[CFG] Relax Wexceptions warning on rethrow

As reported here: https://bugs.llvm.org/show_bug.cgi?id=34973

"catch(...)" should catch EVERYTHING, even a rethrow. This
patch changes the order in which things are checked to ensure
that a '...' catch will get a rethrow.

Differential Revision: https://reviews.llvm.org/D39013

llvm-svn: 316030