clang-tidy explicit constructor check: don't warn on copy or move constructors.

Summary:
http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml#Explicit_Constructors
"The exception is copy constructors, which, in the rare cases when we allow
them, should probably not be explicit."

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3122