Don't refresh stat() info for pcm files

Follow-up fix for 209138.  Actually, since we already have this file
open, we don't want to refresh the stat() info, since that might be
newer than what we have open (bad!).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209143 91177308-0d34-0410-b5e6-96231b3b80d8