[WebAssembly] Handle exception specifications

Summary:
Wasm currently does not fully handle exception specifications. Rather
than crashing, this treats `throw()` in the same way as `noexcept`, and
ignores and prints a warning for `throw(type, ..)`, for a temporary
measure.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79655