Suppress warning on unreachable [[clang::fallthrough]] within a template instantiation.

We don't know whether some other instantiation of the template might be able to
reach the annotation, so warning on it has a high chance of false positives.

Patch by Ahmed Asadi!

Differential Revision: https://reviews.llvm.org/D31069


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298477 91177308-0d34-0410-b5e6-96231b3b80d8