[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/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index c8cefac..790d9857 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -75,9 +75,7 @@
 ToolChain::~ToolChain() {
 }
 
-const Driver &ToolChain::getDriver() const {
- return D;
-}
+vfs::FileSystem &ToolChain::getVFS() const { return getDriver().getVFS(); }
 
 bool ToolChain::useIntegratedAs() const {
   return Args.hasFlag(options::OPT_fintegrated_as,