[VFS] Move RedirectingFileSystem interface into header (NFC)

This moves the RedirectingFileSystem into the header so it can be
extended. This is needed in LLDB we need a way to obtain the external
path to deal with FILE* and file descriptor APIs.

Discussion on the mailing list:
http://lists.llvm.org/pipermail/llvm-dev/2018-November/127755.html

Differential revision: https://reviews.llvm.org/D54277

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351265 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed