[Driver] Add --vfsoverlay flag

[abrachet]

Summary

This flag implies -ivfsoverlay, and additionally passes the same
argument to the linker if it supports it. At present the only linker
which does is lld-link, so this functionality has only been added to
the MSVC toolchain. Additionally this option has been made a
CoreOption so that clang-cl can use it without -Xclang

[phosek]

LGTM

[haowei]

I am seeing test failures on pre submit Windows builder after this change was landed: See https://buildkite.com/llvm-project/premerge-checks/builds/135752#01864d34-9024-4c13-9137-88b6f41ec695

clang/include/clang/Driver/Options.td

 def iwithprefix : JoinedOrSeparate<["-"], "iwithprefix">, Group<clang_i_Group>, Flags<[CC1Option]>,
   HelpText<"Set directory to SYSTEM include search path with prefix">, MetaVarName<"<dir>">;
 def iwithsysroot : JoinedOrSeparate<["-"], "iwithsysroot">, Group<clang_i_Group>,
   HelpText<"Add directory to SYSTEM include search path, "
            "absolute paths are relative to -isysroot">, MetaVarName<"<directory>">,
   Flags<[CC1Option]>;
 def ivfsoverlay : JoinedOrSeparate<["-"], "ivfsoverlay">, Group<clang_i_Group>, Flags<[CC1Option]>,
   HelpText<"Overlay the virtual filesystem described by file over the real file system">;
+def vfsoverlay : JoinedOrSeparate<["-", "--"], "vfsoverlay">, Flags<[CC1Option, CoreOption]>,
+  HelpText<"Overlay the virtual filesystem described by file over the real file system. "
+           "Additionally, pass this overlay file to the linker if it supports it">;
 def imultilib : Separate<["-"], "imultilib">, Group<gfortran_Group>;
 def keep__private__externs : Flag<["-"], "keep_private_externs">;
 def l : JoinedOrSeparate<["-"], "l">, Flags<[LinkerInput, RenderJoined]>,
         Group<Link_Group>;
 def lazy__framework : Separate<["-"], "lazy_framework">, Flags<[LinkerInput]>;
 def lazy__library : Separate<["-"], "lazy_library">, Flags<[LinkerInput]>;
 def mlittle_endian : Flag<["-"], "mlittle-endian">, Flags<[NoXarchOption]>;
 def EL : Flag<["-"], "EL">, Alias<mlittle_endian>;

clang/lib/Driver/ToolChains/Clang.cpp

 
   // Forward -fpass-plugin=name.so to -cc1.
   for (const Arg *A : Args.filtered(options::OPT_fpass_plugin_EQ)) {
     CmdArgs.push_back(
         Args.MakeArgString(Twine("-fpass-plugin=") + A->getValue()));
     A->claim();
   }
 
+  // Forward --vfsoverlay to -cc1.
+  for (const Arg *A : Args.filtered(options::OPT_vfsoverlay)) {
+    CmdArgs.push_back("--vfsoverlay");
+    CmdArgs.push_back(A->getValue());
+    A->claim();
+  }
+
   // Setup statistics file output.
   SmallString<128> StatsFile = getStatsFileName(Args, Output, Input, D);
   if (!StatsFile.empty())
     CmdArgs.push_back(Args.MakeArgString(Twine("-stats-file=") + StatsFile));
 
   // Forward -Xclang arguments to -cc1, and -mllvm arguments to the LLVM option
   // parser.
   for (auto Arg : Args.filtered(options::OPT_Xclang)) {

clang/lib/Driver/ToolChains/MSVC.cpp

void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   llvm::SmallString<128> linkPath;
   StringRef Linker
     = Args.getLastArgValue(options::OPT_fuse_ld_EQ, CLANG_DEFAULT_LINKER);
   if (Linker.empty())
     Linker = "link";
   if (Linker.equals_insensitive("lld"))
     Linker = "lld-link";
 
+  if (Linker == "lld-link")
+    for (Arg *A : Args.filtered(options::OPT_vfsoverlay))
+      CmdArgs.push_back(
+          Args.MakeArgString(std::string("/vfsoverlay:") + A->getValue()));
+
   if (Linker.equals_insensitive("link")) {
     // If we're using the MSVC linker, it's not sufficient to just use link
     // from the program PATH, because other environments like GnuWin32 install
     // their own link.exe which may come first.
     linkPath = FindVisualStudioExecutable(TC, "link.exe");
 
     if (!TC.FoundMSVCInstall() && !canExecute(TC.getVFS(), linkPath)) {
       llvm::SmallString<128> ClPath;

clang/lib/Frontend/CompilerInvocation.cpp

#undef HEADER_SEARCH_OPTION_WITH_MARSHALLING
   }
 
   // Add the path prefixes which are implicitly treated as being system headers.
   for (const auto *A :
        Args.filtered(OPT_system_header_prefix, OPT_no_system_header_prefix))
     Opts.AddSystemHeaderPrefix(
         A->getValue(), A->getOption().matches(OPT_system_header_prefix));
 
-  for (const auto *A : Args.filtered(OPT_ivfsoverlay))
+  for (const auto *A : Args.filtered(OPT_ivfsoverlay, OPT_vfsoverlay))
     Opts.AddVFSOverlayFile(A->getValue());
 
   return Diags.getNumErrors() == NumErrorsBefore;
 }
 
 /// Check if input file kind and language standard are compatible.
 static bool IsInputCompatibleWithStandard(InputKind IK,
                                           const LangStandard &S) {

clang/test/Driver/msvc-link.c

 // REPRO: "-nologo"
 // REPRO: "-Brepro"
 
 // RUN: %clang_cl /Brepro- -fuse-ld=link -### -- %s 2>&1 | FileCheck --check-prefix=NOREPRO %s
 // NOREPRO: link.exe"
 // NOREPRO: "-out:msvc-link.exe"
 // NOREPRO: "-nologo"
 // NOREPRO-NOT: "-Brepro"
+
+// RUN: %clang_cl -fuse-ld-lld --vfsoverlay %s -### -- %s 2>&1 | FileCheck --check-prefix=VFSOVERLAY %s
+// VFSOVERLAY: -cc1"
+// VFSOVERLAY: "--vfsoverlay"
+// VFSOVERLAY: lld-link"
+// VFSOVERLAY: "/vfsoverlay:

clang/test/Driver/vfsoverlay.c

 // RUN: %clang -ivfsoverlay foo.h -### %s 2>&1 | FileCheck %s
 // CHECK: "-ivfsoverlay" "foo.h"
 
+// RUN: %clang --vfsoverlay foo.h -### %s 2>&1 | FileCheck %s
+// CHECK: "--vfsoverlay" "foo.h"
+
 // RUN: not %clang -ivfsoverlay foo.h %s 2>&1 | FileCheck -check-prefix=CHECK-MISSING %s
 // CHECK-MISSING: virtual filesystem overlay file 'foo.h' not found