[Macros on Darwin] Use device platform paths when building for the simulator

To avoid having duplicated macro implementations in both the device and
simulator platforms, when building for a simulator platform, pass
`-external-plugin-path` that points into the *device* platform. There
is no need to have macro implementations in the simulator platforms.

Implements rdar://112563633

Author: DougGregor

Sources/SwiftDriver/Toolchains/DarwinToolchain.swift

@@ -434,13 +434,23 @@ public final class DarwinToolchain: Toolchain {
       commandLine.appendFlag(.externalPluginPath)
       commandLine.appendFlag("\(sdkPathRoot.localPluginPath.name)#\(sdkPathRoot.pluginServerPath.name.spm_shellEscaped())")
 
-      // Default paths for compiler plugins within the platform (accessed via that
-      // platform's plugin server).
-      let platformPathRoot = VirtualPath.lookup(sdkPath)
+      // Determine the platform path. For simulator platforms, look into the
+      // corresponding device platform instance.
+      let origPlatformPath = VirtualPath.lookup(sdkPath)
         .parentDirectory
         .parentDirectory
         .parentDirectory
-        .appending(components: "Developer", "usr")
+      let platformPath: VirtualPath
+      if let simulatorRange = origPlatformPath.basename.range(of: "Simulator.platform") {
+        let devicePlatform = origPlatformPath.basename.replacingCharacters(in: simulatorRange, with: "OS.platform")
+        platformPath = origPlatformPath.parentDirectory.appending(component: devicePlatform)
+      } else {
+        platformPath = origPlatformPath
+      }
+
+      // Default paths for compiler plugins within the platform (accessed via that
+      // platform's plugin server).
+      let platformPathRoot = platformPath.appending(components: "Developer", "usr")
       commandLine.appendFlag(.externalPluginPath)
       commandLine.appendFlag("\(platformPathRoot.pluginPath.name)#\(platformPathRoot.pluginServerPath.name.spm_shellEscaped())")
 

TestInputs/PlatformChecks/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/SDKSettings.json

@@ -0,0 +1,4 @@
+{
+	"Version": "13.0",
+	"CanonicalName": "iphoneos13.0"
+}

TestInputs/PlatformChecks/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/SDKSettings.json

@@ -0,0 +1,4 @@
+{
+	"Version": "15.0",
+	"CanonicalName": "iphonesimulator15.0"
+}

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -6869,7 +6869,13 @@ final class SwiftDriverTests: XCTestCase {
   }
 
   func testPluginPaths() throws {
-    let sdkRoot = testInputsPath.appending(component: "SDKChecks").appending(component: "iPhoneOS.sdk")
+    try pluginPathTest(platform: "iPhoneOS", searchPlatform: "iPhoneOS")
+    try pluginPathTest(platform: "iPhoneSimulator", searchPlatform: "iPhoneOS")
+  }
+
+  func pluginPathTest(platform: String, searchPlatform: String) throws {
+    let sdkRoot = testInputsPath.appending(
+      components: ["PlatformChecks", "\(platform).platform", "Developer", "SDKs", "\(platform).sdk"])
     var driver = try Driver(args: ["swiftc", "-typecheck", "foo.swift", "-sdk", VirtualPath.absolute(sdkRoot).name, "-plugin-path", "PluginA", "-external-plugin-path", "PluginB#Bexe", "-load-plugin-library", "PluginB2", "-plugin-path", "PluginC"])
     guard driver.isFrontendArgSupported(.pluginPath) && driver.isFrontendArgSupported(.externalPluginPath) else {
       return
@@ -6909,7 +6915,11 @@ final class SwiftDriverTests: XCTestCase {
     XCTAssertNotNil(sdkLocalPluginPathIndex)
     XCTAssertLessThan(sdkPluginPathIndex!, sdkLocalPluginPathIndex!)
 
-    let platformPath = sdkRoot.parentDirectory.parentDirectory.parentDirectory.appending(components: "Developer", "usr")
+    let origPlatformPath =
+      sdkRoot.parentDirectory.parentDirectory.parentDirectory.parentDirectory
+        .appending(component: "\(searchPlatform).platform")
+
+    let platformPath = origPlatformPath.appending(components: "Developer", "usr")
     let platformServerPath = platformPath.appending(components: "bin", "swift-plugin-server").pathString
 
     let platformPluginPath = platformPath.appending(components: "lib", "swift", "host", "plugins")