[SwiftCaching] Don't drop -Xcc during dependency scanning

When using swift caching, `-Xcc` should not be appended to compilation
commmands because dependency scanner returns the cc1 command needed to
create clang importer. But `-Xcc` options were accidentally dropped from
scan-dependencies command, causing them to be ignored.

rdar://119391228

Author: cachemeifyoucan

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -387,7 +387,8 @@ extension Driver {
     // Pass through any subsystem flags.
     try commandLine.appendAll(.Xllvm, from: &parsedOptions)
 
-    if !(isCachingEnabled && useClangIncludeTree) {
+    // If using clang-include-tree, `-Xcc` should only be passed to scanDependencies job.
+    if (kind == .scanDependencies) || !(isCachingEnabled && useClangIncludeTree) {
       try commandLine.appendAll(.Xcc, from: &parsedOptions)
     }
 

Tests/SwiftDriverTests/CachingBuildTests.swift

@@ -599,6 +599,10 @@ final class CachingBuildTests: XCTestCase {
       try localFileSystem.writeFileContents(main) {
         $0.send("import C;import E;import G;")
       }
+      let vfsoverlay = path.appending(component: "overlay.yaml")
+      try localFileSystem.writeFileContents(vfsoverlay) {
+        $0.send("{\"case-sensitive\":\"false\",\"roots\":[],\"version\":0}")
+      }
 
       let cHeadersPath: AbsolutePath =
           try testInputsPath.appending(component: "ExplicitModuleBuilds")
@@ -615,6 +619,7 @@ final class CachingBuildTests: XCTestCase {
                                      "-explicit-module-build",
                                      "-cache-compile-job", "-cas-path", casPath.nativePathString(escaped: true),
                                      "-working-directory", path.nativePathString(escaped: true),
+                                     "-Xcc", "-ivfsoverlay", "-Xcc", vfsoverlay.nativePathString(escaped: true),
                                      "-disable-clang-target",
                                      main.nativePathString(escaped: true)] + sdkArgumentsForTesting,
                               env: ProcessEnv.vars)
@@ -642,6 +647,10 @@ final class CachingBuildTests: XCTestCase {
       XCTAssertFalse(scannerCommand.contains("-pch-output-dir"))
       XCTAssertFalse(scannerCommand.contains("Foo.o"))
 
+      // Xcc commands are used for scanner command.
+      XCTAssertTrue(scannerCommand.contains("-Xcc"))
+      XCTAssertTrue(scannerCommand.contains("-ivfsoverlay"))
+
       // Here purely to dump diagnostic output in a reasonable fashion when things go wrong.
       let lock = NSLock()