[VFS] Port driver tool chains to VFS.

There are still some loose ends here but it's sufficient so we can detect
GCC headers that are inside of a VFS.

llvm-svn: 249556
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 649f907..26e767f 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -5955,8 +5955,7 @@
   const std::string MarchSuffix = "/" + MarchString;
   const std::string G0Suffix = "/G0";
   const std::string MarchG0Suffix = MarchSuffix + G0Suffix;
-  const std::string RootDir =
-      toolchains::HexagonToolChain::GetGnuDir(D.InstalledDir, Args) + "/";
+  const std::string RootDir = ToolChain.GetGnuDir(D.InstalledDir, Args) + "/";
   const std::string StartFilesDir =
       RootDir + "hexagon/lib" + (useG0 ? MarchG0Suffix : MarchSuffix);