Diagnostics related to redefinition errors that point to the same header
file do not provide much information that helps users fixing the issue.
- In the modules context, it usually happens because of non modular
includes.
- When modules aren't involved it might happen because of the lack of
header guards.
Enhance diagnostics in these scenarios.
Differential Revision: https://reviews.llvm.org/D28832
rdar://problem/31669175
llvm-svn: 302765
Bruno Cardoso Lopes