Fix assertion in google-explicit-constructor check when the constructor is

defined in a macro.

Summary:
We shouldn't suggest replacements in macros anyway, as we can't see all
usages of the macro and ensure the replacement is safe for all of them.

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3611

llvm-svn: 207987