[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`.
- This ignores and prints a warning for `throw(type, ..)`, for a
  temporary measure. This warning is controlled by
  `-Wwasm-exception-spec`, which is on by default. You can suppress the
  warning by using `-Wno-wasm-exception-spec`.

Reviewers: dschuff

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80061