Improve the error message when a PCH dependency is modified

Show the top-level pch file as the culprit, rather than the immediate
dependency when a pch file imports a pcm from a module. To clarify the
relationship, the pch import stack is printed as notes. The old behaviour was
misleading when a pch imported a pcm (from a module), since removing the pcm
would not fix the problem, whereas rebuilding the pch would.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199446 91177308-0d34-0410-b5e6-96231b3b80d8