Test for warning being silenced

Summary:
Added some tests to see if the new warning would be silenced with a flag.

Patch by Arthur Marble <arthur@info9.net> in the context of Debian Google Summer of code 2014.


Reviewers: sylvestre.ledru

Reviewed By: sylvestre.ledru

Differential Revision: http://reviews.llvm.org/D4475

llvm-svn: 212833