[VFS] Wire up driver VFS through tooling.
[VFS] Wire up driver VFS through tooling.

Sadly I don't currently have a way to tests this as the driver is always
initialized with the default triple and finding system headers is system
specific.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@249831 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang