sil-opt: introduce -vfsoverlay to support Windows

Add an option to `sil-opt` to pass along VFS overlay files to the clang
importer.  This is required to make the tests pass with the upcoming
work to avoid modifying the Visual Studio installation and instead to
prefer to inject the content via VFS overlays.

Author: compnerd

include/swift/Frontend/Frontend.h

@@ -188,6 +188,10 @@ class CompilerInvocation {
     return SearchPathOpts.getFrameworkSearchPaths();
   }
 
+  void setVFSOverlays(const std::vector<std::string> &Overlays) {
+    SearchPathOpts.VFSOverlayFiles = Overlays;
+  }
+
   void setCompilerPluginLibraryPaths(const std::vector<std::string> &Paths) {
     SearchPathOpts.setCompilerPluginLibraryPaths(Paths);
   }

tools/sil-opt/SILOpt.cpp

@@ -86,6 +86,9 @@ ImportPaths("I", llvm::cl::desc("add a directory to the import search path"));
 static llvm::cl::list<std::string>
 FrameworkPaths("F", llvm::cl::desc("add a directory to the framework search path"));
 
+static llvm::cl::list<std::string>
+VFSOverlays("vfsoverlay", llvm::cl::desc("add a VFS overlay"));
+
 static llvm::cl::opt<std::string>
 ModuleName("module-name", llvm::cl::desc("The name of the module if processing"
                                          " a module. Necessary for processing "
@@ -542,6 +545,9 @@ int main(int argc, char **argv) {
     FramePaths.push_back({path, /*isSystem=*/false});
   }
   Invocation.setFrameworkSearchPaths(FramePaths);
+
+  Invocation.setVFSOverlays(VFSOverlays);
+
   // Set the SDK path and target if given.
   if (SDKPath.getNumOccurrences() == 0) {
     const char *SDKROOT = getenv("SDKROOT");