Propagate vfs overlays and sdk to subinvocations

lib/ClangImporter/ClangImporter.cpp

@@ -1411,6 +1411,11 @@ ClangImporter::create(ASTContext &ctx,
           importer->Impl.getBufferImporterForDiagnostics(),
           importer->Impl.platformAvailability, &importer->Impl));
 
+  // Pass along the VFS overlays
+  for (auto &File : ctx.SearchPathOpts.VFSOverlayFiles) {
+    importer->Impl.Invocation->getHeaderSearchOpts().VFSOverlayFiles.push_back(File);
+  }
+
   // Create a compiler instance.
   {
     // The Clang modules produced by ClangImporter are always embedded in an

lib/ClangImporter/ClangModuleDependencyScanner.cpp

@@ -112,6 +112,14 @@ static std::vector<std::string> getClangDepScanningInvocationArguments(
   commandLineArgs.push_back("-Xclang");
   commandLineArgs.push_back("-finclude-tree-preserve-pch-path");
 
+  // Handle VFSOverlay. If include tree is used, there is no need for overlay.
+  if (!ctx.ClangImporterOpts.UseClangIncludeTree) {
+    for (auto &overlay : ctx.SearchPathOpts.VFSOverlayFiles) {
+      commandLineArgs.push_back("-vfsoverlay");
+      commandLineArgs.push_back(overlay);
+    }
+  }
+
   return commandLineArgs;
 }
 
@@ -185,6 +193,13 @@ ModuleDependencyVector ClangImporter::bridgeClangModuleDependencies(
         swiftArgs.push_back("-vfsoverlay");
         swiftArgs.push_back(remapPath(overlay));
       }
+
+      // Pass along the SDK path
+      StringRef SDKPath = ctx.SearchPathOpts.getSDKPath();
+      if (!SDKPath.empty()) {
+        swiftArgs.push_back("-sdk");
+        swiftArgs.push_back(SDKPath.str());
+      }
     }
 
     // Add args reported by the scanner.

test/ScanDependencies/hello-explicit-module-build.swift

@@ -0,0 +1,7 @@
+// RUN: %target-run-simple-swift | %FileCheck %s
+
+// REQUIRES: executable_test
+
+print("Hello, Explicit Module Build!")
+
+// CHECK: Hello, Explicit Module Build!

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"