Stat system dependencies when using -verify-pch

We don't stat the system headers to check for stalenes during regular
PCH loading for performance reasons.  When explicitly saying
-verify-pch, we want to check all the dependencies - user or system.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200979 91177308-0d34-0410-b5e6-96231b3b80d8