Warn on explicit copy constructors.

Summary:
The Google C++ Style Guide doesn't require copy constructors to be
declared explicit, but some people do this by mistake. Make this check detect
and fix such cases.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3541

llvm-svn: 207531