[FileManager] Do not call 'real_path' in getFile().

Summary:
This partially rolls back the change in D48903:
https://github.com/llvm-mirror/clang/commit/89aa7f45a1f728144935289d4ce69d8522999de0#diff-0025af005307891b5429b6a834823d5eR318

`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):
Before
{F7039629}
After
{F7039630}

Reviewers: ilya-biryukov, simark

Reviewed By: ilya-biryukov

Subscribers: kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D51159

llvm-svn: 340598