Improve our uniquing of file entries when files are re-saved or are

Improve our uniquing of file entries when files are re-saved or are
overridden via remapping. Thus, when we create a "virtual" file in the
file manager, we still stat() the real file that lives behind it so
that we can provide proper uniquing based on inodes. This helps keep
the file manager much more consistent.

To take advantage of this when reparsing files in libclang, we disable
the use of the stat() cache when reparsing or performing code
completion, since the stat() cache is very likely to be out of date in
this use case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124971 91177308-0d34-0410-b5e6-96231b3b80d8