Propagate vfs overlays and -fbuiltin-headers-in-system-modules

This fixes explicit module builds for a hello world program on Windows
as well as the ucrt import build failure as in the included test.

Author: hjyamauchi

lib/ClangImporter/ClangImporter.cpp

@@ -957,6 +957,18 @@ importer::addCommonInvocationArguments(
       }
     }
   }
+
+  for (auto &overlay : searchPathOpts.VFSOverlayFiles) {
+    invocationArgStrs.push_back("-ivfsoverlay");
+    invocationArgStrs.push_back(overlay);
+  }
+
+  ClangInvocationFileMapping fileMapping =
+      getClangInvocationFileMapping(ctx, nullptr, /*supressDiagnostic*/true);
+  if (fileMapping.requiresBuiltinHeadersInSystemModules) {
+    invocationArgStrs.push_back("-Xclang");
+    invocationArgStrs.push_back("-fbuiltin-headers-in-system-modules");
+  }
 }
 
 bool ClangImporter::canReadPCH(StringRef PCHFilename) {

test/ScanDependencies/win-crt.swift

@@ -0,0 +1,11 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -scan-dependencies -Xcc -v %s -o - | %validate-json | %FileCheck %s
+
+// We want to explicitly import WinSDK's CRT.
+// REQUIRES: OS=windows-msvc
+
+import CRT
+
+// CHECK:        "modulePath": "{{.*}}\\ucrt-{{.*}}.pcm",
+// CHECK-NEXT:   "sourceFiles": [
+// CHECK-NEXT:      "{{.*}}\\ucrt\\module.modulemap"