[PCH+Modules] Improve diagnosticts to help out users pass an extra header search path

When mixing PCH and Implicit Modules, missing a header search path
can lead to the implicit built PCM to complaint about not finding its
matching module map.

Instead of adding more magic to implicit modules engine, add a note to
help the user add the appropriate path.

rdar://problem/33388847

llvm-svn: 318503