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

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@201308 91177308-0d34-0410-b5e6-96231b3b80d8