Frontend: Split out addVFSOverlays from createVFSFromCompilerInvocation, NFC

[dexonsmith]

Summary

This is a prep patch to reduce noise in a follow-up, which will add
another layer.

[jansvoboda11]

LGTM

clang/lib/Frontend/CompilerInvocation.cpp

 
 IntrusiveRefCntPtr<llvm::vfs::FileSystem>
 clang::createVFSFromCompilerInvocation(const CompilerInvocation &CI,
                                        DiagnosticsEngine &Diags) {
   return createVFSFromCompilerInvocation(CI, Diags,
                                          llvm::vfs::getRealFileSystem());
 }
 
-IntrusiveRefCntPtr<llvm::vfs::FileSystem>
-clang::createVFSFromCompilerInvocation(
-    const CompilerInvocation &CI, DiagnosticsEngine &Diags,
+static IntrusiveRefCntPtr<llvm::vfs::FileSystem>
+addVFSOverlays(const CompilerInvocation &CI, DiagnosticsEngine &Diags,
                IntrusiveRefCntPtr<llvm::vfs::FileSystem> BaseFS) {
   if (CI.getHeaderSearchOpts().VFSOverlayFiles.empty())
     return BaseFS;
 
   IntrusiveRefCntPtr<llvm::vfs::FileSystem> Result = BaseFS;
   // earlier vfs files are on the bottom
   for (const auto &File : CI.getHeaderSearchOpts().VFSOverlayFiles) {
     llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Buffer =
         Result->getBufferForFile(File);
if (!FS) {
       Diags.Report(diag::err_invalid_vfs_overlay) << File;
       continue;
     }
 
     Result = FS;
   }
   return Result;
 }
+
+IntrusiveRefCntPtr<llvm::vfs::FileSystem>
+clang::createVFSFromCompilerInvocation(
+    const CompilerInvocation &CI, DiagnosticsEngine &Diags,
+    IntrusiveRefCntPtr<llvm::vfs::FileSystem> BaseFS) {
+  IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS = std::move(BaseFS);
+  FS = addVFSOverlays(CI, Diags, std::move(FS));
+  return FS;
+}