Extend stat query APIs to explicitly specify if the query is for

Extend stat query APIs to explicitly specify if the query is for
a file or directory, allowing just a stat call if a file descriptor
is not needed.

Doing just 'stat' is faster than 'open/fstat/close'.
This has the effect of cutting down system time for validating the input files of a PCH.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169831 91177308-0d34-0410-b5e6-96231b3b80d8