[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 ]].

Reviewers: dschuff

Subscribers: jfb, sbc100, llvm-commits

Differential Revision: https://reviews.llvm.org/D43706

llvm-svn: 326985