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.