[WebAssembly] -fwasm-exceptions enables reference-types

Summary:
This adds `-mreference-types` and `-mno-reference-types` flags to clang
and make `-fwasm-exceptions` enables reference types feature in clang
and the backend.

Reviewers: tlively

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69832