[Driver] Support libc++ in MSVC

This implements support for using libc++ headers and library in the MSVC
toolchain.  We only support libc++ that is a part of the toolchain, and
not headers installed elsewhere on the system.

Differential Revision: https://reviews.llvm.org/D101479

clang/lib/Driver/ToolChains/MSVC.cpp

@@ -435,6 +435,11 @@ void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   if (!C.getDriver().IsCLMode() && Args.hasArg(options::OPT_L))
     for (const auto &LibPath : Args.getAllArgValues(options::OPT_L))
       CmdArgs.push_back(Args.MakeArgString("-libpath:" + LibPath));
+  // Add library directories for standard library shipped with the toolchain.
+  for (const auto &LibPath : TC.getFilePaths()) {
+    if (TC.getVFS().exists(LibPath))
+      CmdArgs.push_back(Args.MakeArgString("-libpath:" + LibPath));
+  }
 
   CmdArgs.push_back("-nologo");
 
@@ -1323,7 +1328,36 @@ void MSVCToolChain::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
 
 void MSVCToolChain::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
                                                  ArgStringList &CC1Args) const {
-  // FIXME: There should probably be logic here to find libc++ on Windows.
+  if (DriverArgs.hasArg(options::OPT_nostdlibinc) ||
+      DriverArgs.hasArg(options::OPT_nostdincxx))
+    return;
+
+  switch (GetCXXStdlibType(DriverArgs)) {
+  case ToolChain::CST_Libcxx: {
+    SmallString<128> P(getDriver().Dir);
+    llvm::sys::path::append(P, "..", "include");
+
+    std::string Version = detectLibcxxVersion(P);
+    if (Version.empty())
+      return;
+
+    // First add the per-target include path if it exists.
+    SmallString<128> TargetDir(P);
+    llvm::sys::path::append(TargetDir, getTripleString(), "c++", Version);
+    if (getVFS().exists(TargetDir))
+      addSystemInclude(DriverArgs, CC1Args, TargetDir);
+
+    // Second add the generic one.
+    SmallString<128> Dir(P);
+    llvm::sys::path::append(Dir, "c++", Version);
+    addSystemInclude(DriverArgs, CC1Args, Dir);
+    break;
+  }
+
+  default:
+    // TODO: Shall we report an error for other C++ standard libraries?
+    break;
+  }
 }
 
 VersionTuple MSVCToolChain::computeMSVCVersion(const Driver *D,

clang/test/Driver/msvc-libcxx.cpp

@@ -0,0 +1,7 @@
+// RUN: %clangxx -### %s 2>&1 -stdlib=libc++ -fuse-ld=lld \
+// RUN:   --target=x86_64-pc-windows-msvc \
+// RUN:   -ccc-install-dir %S/Inputs/msvc_libcxx_tree/usr/bin \
+// RUN:   | FileCheck %s -check-prefix MSVC-LIBCXX
+// MSVC-LIBCXX: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}x86_64-pc-windows-msvc{{/|\\\\}}c++{{/|\\\\}}v1"
+// MSVC-LIBCXX: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}c++{{/|\\\\}}v1"
+// MSVC-LIBCXX: "-libpath:{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-pc-windows-msvc"