[PCH] When validating that the files coming from PCH did not change, also

[PCH] When validating that the files coming from PCH did not change, also
validate that we didn't override the contents of any of such files.

If this is detected, emit a diagnostic error and recover gracefully
by using the contents of the original file that the PCH was built from.

Part of rdar://11305263

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156107 91177308-0d34-0410-b5e6-96231b3b80d8