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

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@207987 91177308-0d34-0410-b5e6-96231b3b80d8