Per discussion on cfe-dev, re-enable suppression of -Wimplicit-fallthrough on...

Per discussion on cfe-dev, re-enable suppression of -Wimplicit-fallthrough on C, but also include dialects of C++ earlier than C++11.

There was enough consensus that we *can* get a good language solution
to have an annotation outside of C++11, and without this annotation
this warning doesn't quite mean's completeness criteria for this
kind of warning.  For now, restrict this warning to C++11 (where an
annotation exists), and make this the behavior for the LLVM 3.2 release.
Afterwards, we will hammer out a language solution that we are all
happy with.

llvm-svn: 167749