[VFS] Add a proxy FS that delegates calls to underlying FS by default.
[VFS] Add a proxy FS that delegates calls to underlying FS by default.

Summary:
This is useful when derived file systems want to override some calls
and still proxy other calls.

Reviewers: ilya-biryukov, sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D52462

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@342976 91177308-0d34-0410-b5e6-96231b3b80d8

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