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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212833 91177308-0d34-0410-b5e6-96231b3b80d8