Honor attribute 'analyzer_noreturn' on Objective-C methods.

This isn't likely a full solution, but it catches the common cases
and can be refined over time.

Fixes <rdar://problem/11634353>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173291 91177308-0d34-0410-b5e6-96231b3b80d8