VFS: Avoid some unnecessary std::string copies

VFS: Avoid some unnecessary std::string copies

Thread Twine a little deeper through the VFS to avoid unnecessarily
constructing the same std::string twice in a parameter sequence:

    Twine -> std::string -> StringRef -> std::string

Changing a few parameters from StringRef to Twine avoids the early call
to `Twine::str()`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354739 91177308-0d34-0410-b5e6-96231b3b80d8