PackageModel: adjust toolchain layout for Windows

Add in a migration path for adjusting the layout of the toolchain.  This
is required in order to support more than one architecture SDK to be
installed simultaneously.  The problem is primarily on the XCTest side,
where the binary is not slotted to an architecture.  Having multiple
variants of XCTest is difficult with the toolchain layout.  The XCTest
installation is located at
`%DEVELOPER_DIR%\Platforms\<Platform>.platform\Library\XCTest-<version>`.
The runtime component is located under, and the DLL is always named
`XCTest.dll`.  The following options were considered:

1. renaming `XCTest.dll` to:
  - `XCTest32.dll` (x86)
  - `XCTest64.dll` (x64)
  - `XCTest32a.dll` (ARMv7)
  - `XCTest64a.dll` (ARM64)

2. renaming `bin` to:
  - `bin32` (x86)
  - `bin64` (x64)
  - `bin32a` (ARMv7)
  - `bin64a` (ARM64)

3. adding a subdirectory to `bin`

Option 2 was deemed to be the best of the terrible options.  It would
allow a migration path (as the DLL name is embedded into the import
library and subsequently the executable).

We simultaneously update the location for the import library.  Although
it is possible to build a multi-architecture import library, we would
need generate that manually before the installer package is built and
then only install the runtime.  It seems that moving the directory would
be easier and ensure that we can keep them somewhat independent.

Author: compnerd

Sources/PackageModel/UserToolchain.swift

@@ -255,11 +255,28 @@ public final class UserToolchain: Toolchain {
                             root.appending(component: "Developer")
                                 .appending(component: "Library")
                                 .appending(component: "XCTest-\(info.defaults.xctestVersion)")
+
                             xctest = [
                                 "-I", AbsolutePath("usr/lib/swift/windows/\(triple.arch)", relativeTo: path).pathString,
-                                "-L", AbsolutePath("usr/lib/swift/windows", relativeTo: path).pathString,
+                                "-L", AbsolutePath("usr/lib/swift/windows/\(triple.arch)", relativeTo: path).pathString,
                             ]
 
+                            // Migration Path
+                            //
+                            // In order to support multiple parallel
+                            // installations of an SDK, we need to ensure that
+                            // we can have all the architecture variant
+                            // libraries available.  Prior to this getting
+                            // enabled (~5.7), we always had a singular
+                            // installed SDK.  Prefer the new variant which has
+                            // an architecture subdirectory in `bin` if
+                            // available.
+                            let implib: AbsolutePath =
+                                AbsolutePath("usr/lib/swift/windows/XCTest.lib", relativeTo: path)
+                            if localFileSystem.exists(implib) {
+                                xctest.append(contentsOf: ["-L", implib.parentDirectory.pathString])
+                            }
+
                             extraSwiftCFlags = info.defaults.extraSwiftCFlags ??  []
                         }
                     }
@@ -460,11 +477,63 @@ public final class UserToolchain: Toolchain {
                 if let info = WindowsPlatformInfo(reading: root.appending(component: "Info.plist"),
                                                   diagnostics: nil,
                                                   filesystem: localFileSystem) {
-                    return root.appending(component: "Developer")
-                        .appending(component: "Library")
-                        .appending(component: "XCTest-\(info.defaults.xctestVersion)")
-                        .appending(component: "usr")
-                        .appending(component: "bin")
+
+                    let installation: AbsolutePath =
+                        root.appending(component: "Developer")
+                            .appending(component: "Library")
+                            .appending(component: "XCTest-\(info.defaults.xctestVersion)")
+
+                    // Migration Path
+                    //
+                    // In order to support multiple parallel installations of an
+                    // SDK, we need to ensure that we can have all the
+                    // architecture variant libraries available.  Prior to this
+                    // getting enabled (~5.7), we always had a singular
+                    // installed SDK.  Prefer the new variant which has an
+                    // architecture subdirectory in `bin` if available.
+                    switch triple.arch {
+                    case .x86_64, .x86_64h:
+                        let path: AbsolutePath =
+                            installation.appending(component: "usr")
+                                        .appending(component: "bin64")
+                        if localFileSystem.exists(path) {
+                            return path
+                        }
+
+                    case .i686:
+                        let path: AbsolutePath =
+                            installation.appending(component: "usr")
+                                        .appending(component: "bin32")
+                        if localFileSystem.exists(path) {
+                            return path
+                        }
+
+                    case .armv7:
+                        let path: AbsolutePath =
+                            installation.appending(component: "usr")
+                                        .appending(component: "bin32a")
+                        if localFileSystem.exists(path) {
+                            return path
+                        }
+
+                    case .arm64:
+                        let path: AbsolutePath =
+                            installation.appending(component: "usr")
+                                        .appending(component: "bin64a")
+                        if localFileSystem.exists(path) {
+                            return path
+                        }
+
+                    default:
+                        // Fallback to the old-style layout.  We should really
+                        // report an error in this case - this architecture is
+                        // unavailable.
+                        break
+                    }
+
+                    // Assume that we are in the old-style layout.
+                    return installation.appending(component: "usr")
+                                       .appending(component: "bin")
                 }
             }
         }