In openFileForRead don't cache erroneous entries if the error relates to them...

In openFileForRead don't cache erroneous entries if the error relates to them being directories. Add tests.

Summary:
It seems that when the CachingFileSystem is first given a file to open that is actually a directory, it incorrectly
caches that path to be errenous and throws an error when subsequently a directory open call is made for the same
path.
This change makes it so that we do NOT cache a path if it turns out we asked for a file when its a directory.

Reviewers: arphaman

Subscribers: dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68193

llvm-svn: 374366