Split out llvm/Support/FileSystem/UniqueID.h and clang/Basic/FileEntry.h, NFC

Split `FileEntry` and `FileEntryRef` out into a new file
`clang/Basic/FileEntry.h`. This allows current users of a
forward-declared `FileEntry` to transition to `FileEntryRef` without
adding more includers of `FileManager.h`.

Also split `UniqueID` out to llvm/Support/FileSystem/UniqueID.h, so
`FileEntry.h` doesn't need to include all of `FileSystem.h` for just
that type.

Differential Revision: https://reviews.llvm.org/D89761