FileManager: Do not cache failed stats, it is easy to construct common

inconsistent situations if we do, and they are not important for PCH performance
(which currently only needs the stats to construct the initial FileManager
entries).
 - No test case, sorry, the machinations are too involved.

This occurs when, for example, the build makes a PCH file and has a header map
or a -I for a directory that does not yet exist. It is possible we will cache
the negative stat on that directory, and then in the build we will never find
header files inside that dir.

For PCH we don't need these stats anyway for performance, so this also makes PCH
files smaller w/ no loss. I hope to eventually eliminate the stat cache
entirely.

llvm-svn: 91082