[WebAssembly] Hide new Wasm EH behind its feature flag

Summary:
clang's current wasm EH implementation is a non-MVP feature in progress.
We had a `-mexception-handling` wasm feature but were not using it. This
patch hides the non-MVP wasm EH behind a flag, so it does not affect
other code for now.

Reviewers: dschuff

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

Differential Revision: https://reviews.llvm.org/D47614

llvm-svn: 333716