<rdar://problem/13643854> Only emit ambiguous-expansion warnings when at least one of the macro definitions comes from a non-system header.

This slightly weakens the heuristic introduced in r178109.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179411 91177308-0d34-0410-b5e6-96231b3b80d8