[Frontend] Use vfs for directory iteration while searching PCHs. NFCI
[Frontend] Use vfs for directory iteration while searching PCHs. NFCI

Use the vfs lookup instead of real filesytem and handle the case where
-include-pch is a directory and this dir is searched for a PCH.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@289459 91177308-0d34-0410-b5e6-96231b3b80d8

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