[clangd] Make PreambleStatusCache handle filenames more carefully

Summary:
 - when we hit the cache, the reported filename should be that of the
   cache query, not that of the cache store. This matches behaviors of
   common FSes, and avoids triggering difficult edge cases in
   FileManager when files are being moved around concurrently.
 - filename comparisons (both cache queries and == mainfile checks)
   should fold away . and .. in paths. These can appear when relative
   paths occur in compile_commands.json. (gn does this).

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63931

llvm-svn: 364740