Fix ExplicitConstructorCheck to warn only on in-class declarations.

Summary:
I'm not absolutely sure this is 100% correct solution, but it seems to
do what I expect.

Reviewers: djasper, klimek

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2756

llvm-svn: 201308