Move google-explicit-constructor check to a separate source file.

Summary: No functional changes.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4188

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@211154 91177308-0d34-0410-b5e6-96231b3b80d8