[analyzer] Treat throws as sinks for suppress-on-sink purposes.

Because since r308957 the suppress-on-sink feature contains its own
mini-analysis, it also needs to become aware that C++ unhandled exceptions
cause sinks. Unfortunately, for now we treat all exceptions as unhandled in
the analyzer, so suppress-on-sink needs to do the same.

rdar://problem/28157554

Differential Revision: https://reviews.llvm.org/D35674

llvm-svn: 308961