Add a CC1 option -verify-pch

This option will:
- load the given pch file
- verify it is not out of date by stat'ing dependencies, and
- return 0 on success and non-zero on error

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@200884 91177308-0d34-0410-b5e6-96231b3b80d8