Propagate vfs overlays and sdk to subinvocations

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 +957,11 @@ importer::addCommonInvocationArguments(
       }
     }
   }
+
+  for (auto &overlay : searchPathOpts.VFSOverlayFiles) {
+    invocationArgStrs.push_back("-ivfsoverlay");
+    invocationArgStrs.push_back(overlay);
+  }
 }
 
 bool ClangImporter::canReadPCH(StringRef PCHFilename) {

lib/ClangImporter/ClangModuleDependencyScanner.cpp

@@ -185,6 +185,13 @@ 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/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"