Added a test for C++11 statement attributes serialization.

Summary: Uses [[clang::fallthrough]] attribute in a template function, and -Wimplicit-fallthrough to check the attribute presence in an instantiation.

Reviewers: rsmith

Reviewed By: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D55

llvm-svn: 165068