Previews break with explicit modules enabled <rdar://146184835>

With the new previews execution mode, we don't need to be as aggressive stripping compiler flags for thunk builds like we did for the old dynamic replacement execution mode. And doing so actually breaks when explicit modules are turned on. After auditing the minimum number of flags needed to request single file compiler invocations that don't break with explict modules enabled, this change moves the remaining flags into clauses that only affect dynamic replacement mode.

Updated unit tests to match (including some better formatting for the large parameter checks).

Author: jonathanpenn

Sources/SWBCore/SpecImplementations/Tools/SwiftCompiler.swift

@@ -3241,42 +3241,42 @@ public final class SwiftCompilerSpec : CompilerSpec, SpecIdentifierType, SwiftDi
             // Preview thunks does not need this
             "-emit-objc-header",
 
-            // Stripping for completeness, but XOJIT Previews are already forced as `-Onone`.
-            // We add back `-Onone` below.
+            // Previews only work on unoptimized build products.
             "-O",
-            "-Onone",
             "-Osize",
 
             // _Very_ deprecated, but continuing to strip for backwards compatibility
             "-embed-bitcode",
             "-embed-bitcode-marker",
 
-            // Stripped for historical reasons. Previews can add it back if it needs debugging information.
-            "-g",
-            "-dwarf-version=4",
-            "-dwarf-version=5",
-
             // Previews does not use compiler output
             "-parseable-output",
             "-use-frontend-parseable-output",
 
             // Writes more intermediates that Previews does not need
             "-emit-const-values",
             "-save-temps",
-
-            "-explicit-module-build",
-
-            // Strip until builder SDKs include a swift-driver with this flag. Do not remove without also removing -clang-build-session-file.
-            "-validate-clang-modules-once"
         ] {
             while let index = commandLine.firstIndex(of: arg) {
                 commandLine.remove(at: index)
             }
         }
         if payload.previewStyle == .dynamicReplacement {
             for arg in [
+                // Debug symbols aren't needed in dynamic replacement preview info.
+                "-g",
+
+                // We add back `-Onone` below in dynamic replacement.
+                "-Onone",
+
                 // Dynamic replacement thunks don't need this.
                 "-import-underlying-module",
+
+                // This property was always stripped in dynamic replacement.
+                "-explicit-module-build",
+
+                // Strip until builder SDKs include a swift-driver with this flag. Do not remove without also removing -clang-build-session-file.
+                "-validate-clang-modules-once"
             ] {
                 while let index = commandLine.firstIndex(of: arg) {
                     commandLine.remove(at: index)
@@ -3334,17 +3334,17 @@ public final class SwiftCompilerSpec : CompilerSpec, SpecIdentifierType, SwiftDi
         removeWithPrefix("-cache-compile-job")
         commandLine.append("-disable-bridging-pch")
 
-        for (arg, newValue) in [
-            // Previews needs a path to _a_ module cache so it has a place to store built modules.
-            ("-module-cache-path", previewPayload.moduleCacheDir.str)
-        ] {
-            removeWithParameter(arg)
-            if !newValue.isEmpty {
-                commandLine += [arg, newValue]
+        if payload.previewStyle == .dynamicReplacement {
+            for (arg, newValue) in [
+                // Previews needs a path to _a_ module cache so it has a place to store built modules.
+                ("-module-cache-path", previewPayload.moduleCacheDir.str)
+            ] {
+                removeWithParameter(arg)
+                if !newValue.isEmpty {
+                    commandLine += [arg, newValue]
+                }
             }
-        }
 
-        if payload.previewStyle == .dynamicReplacement {
             for arg in [
                 // We want the objc header in XOJIT mode so ignore in dynamic replacement mode
                 "-import-objc-header",
@@ -3414,16 +3414,16 @@ public final class SwiftCompilerSpec : CompilerSpec, SpecIdentifierType, SwiftDi
                 "\(payload.moduleName)_PreviewReplacement_\(sourceFile.basenameWithoutSuffix)_\(thunkVariantSuffix)".mangledToC99ExtendedIdentifier(),
             ])
             commandLine.append("-parse-as-library")
-        }
 
-        // Faster thunk builds
-        commandLine.append("-Onone")
+            // Faster thunk builds
+            commandLine.append("-Onone")
 
-        // Faster thunk builds
-        commandLine.append(contentsOf: [
-            "-Xfrontend",
-            "-disable-modules-validate-system-headers",
-            ])
+            // Faster thunk builds
+            commandLine.append(contentsOf: [
+                "-Xfrontend",
+                "-disable-modules-validate-system-headers",
+                ])
+        }
 
         // For XOJIT previews, we want the frontend (`swift-frontend`) invocation rather than the driver (`swiftc`) invocation, so ask libSwiftDriver for it and replace the command line with the result for propagation back to the request.
         if let driverPayload = payload.driverPayload, payload.previewStyle == .xojit {