[WebAssembly] Add except_ref as a first-class type

Summary:
Add except_ref as a first-class type, according to the [[https://github.com/WebAssembly/exception-handling/blob/master/proposals/Level-1.md | Level 1 exception handling proposal ]].
Companion to D43706.

Reviewers: sbc100

Subscribers: jfb, dschuff, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D43707

llvm-svn: 326986