Running `-fsyntax-only` on UniqueID.h is 2x faster with this patch
(which avoids calling `std::tie` for `operator<`). Since the transitive
includers of this file will go up as `FileEntryRef` gets used in more
places, avoid that compile-time hit. This is a follow-up to
23ed570af1cc165afea1b70a533a4a39d6656501 (suggested by Reid Kleckner).
Also drop the `<tuple>` include from FileSystem.h (which was vestigal
from before UniqueID.h was split out).
Differential Revision: https://reviews.llvm.org/D90471
Not sure why this worked for me, but some of the bots pointed out I
copied the wrong includes from FileSystem.h in
23ed570af1cc165afea1b70a533a4a39d6656501. Fixed.
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
