Revert r163411 based on review discussion.

In code review, it looked like these warnings do not actually fire with
modern Clang or GCC. However, the pragma is flat out rejected by GCC in
many configurations (-Werror) so it ended up causing more problems than
it fixed.

Daniel agreed with reverting this, but I think didn't have time to get
to it.

llvm-svn: 163583