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
23ed570af1 (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
Duncan P. N. Exon Smith