[FileManager] Do not call 'real_path' in getFile().
This partially rolls back the change in D48903:
real_path can be very expensive on real file systems, and calling it on each
opened file can slow down the compilation. This also slows down deserialized
ASTs for which real paths need to be recalculated for each input files again.
For clangd code completion latency (using preamble):
Reviewers: ilya-biryukov, simark
Reviewed By: ilya-biryukov
Subscribers: kadircet, cfe-commits
Differential Revision: https://reviews.llvm.org/D51159