[WebAssembly] Warn on exception spec for Emscripten EH

It turns out we have not correctly supported exception spec all along in
Emscripten EH. Emscripten EH supports `throw()` but not `throw` with
types. See https://bugs.llvm.org/show_bug.cgi?id=50396.

Wasm EH also only supports `throw()` but not `throw` with types, and we
have been printing a warning message for the latter. This prints the
same warning message for `throw` with types when Emscripten EH is used,
or more precisely, when Wasm EH is not used. (So this will print the
warning messsage even when `-fno-exceptions` is used but I think that
should be fine. It's cumbersome to do a complilcated option checking in
CGException.cpp and options checkings are mostly done in elsewhere.)

Reviewed By: dschuff, kripken

Differential Revision: https://reviews.llvm.org/D102791