[Dependency Scanning] Do not include PCH-handling command-line flags on the scanning jobs

These jobs do not require handling of the module's PCH.

Author: artemcm

Sources/SwiftDriver/ExplicitModuleBuilds/ModuleDependencyScanning.swift

@@ -49,7 +49,7 @@ public extension Driver {
     commandLine.appendFlag("-frontend")
     commandLine.appendFlag("-scan-dependencies")
     try addCommonFrontendOptions(commandLine: &commandLine, inputs: &inputs,
-                                 bridgingHeaderHandling: .precompiled,
+                                 bridgingHeaderHandling: .ignored,
                                  moduleDependencyGraphUse: .dependencyScan)
     // FIXME: MSVC runtime flags
 
@@ -224,7 +224,7 @@ public extension Driver {
     commandLine.appendFlag("-scan-dependencies")
     commandLine.appendFlag("-import-prescan")
     try addCommonFrontendOptions(commandLine: &commandLine, inputs: &inputs,
-                                 bridgingHeaderHandling: .precompiled,
+                                 bridgingHeaderHandling: .ignored,
                                  moduleDependencyGraphUse: .dependencyScan)
     // FIXME: MSVC runtime flags
 
@@ -255,7 +255,7 @@ public extension Driver {
     commandLine.appendFlag("-frontend")
     commandLine.appendFlag("-scan-dependencies")
     try addCommonFrontendOptions(commandLine: &commandLine, inputs: &inputs,
-                                 bridgingHeaderHandling: .precompiled,
+                                 bridgingHeaderHandling: .ignored,
                                  moduleDependencyGraphUse: .dependencyScan)
 
     let batchScanInputFilePath = try serializeBatchScanningModuleArtifacts(moduleInfos: moduleInfos)

Tests/SwiftDriverTests/ExplicitModuleBuildTests.swift

@@ -758,6 +758,7 @@ final class ExplicitModuleBuildTests: XCTestCase {
       var driver = try Driver(args: ["swiftc",
                                      "-I", cHeadersPath,
                                      "-I", swiftModuleInterfacesPath,
+                                     "-import-objc-header",
                                      "-experimental-explicit-module-build",
                                      "-working-directory", path.pathString,
                                      "-disable-clang-target",
@@ -772,6 +773,9 @@ final class ExplicitModuleBuildTests: XCTestCase {
         scannerCommand.removeFirst()
       }
 
+      // Ensure we do not propagate the ususal PCH-handling arguments to the scanner invocation
+      XCTAssertFalse(scannerCommand.contains("-pch-output-dir"))
+
       // Here purely to dump diagnostic output in a reasonable fashion when things go wrong.
       let lock = NSLock()