[WebAssembly] Add '.eventtype' directive support

Summary:
This patch supports `.eventtype` directive printing and parsing in the
same syntax with `.functype`.

Reviewers: aardappel, sbc100

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

Differential Revision: https://reviews.llvm.org/D55353

llvm-svn: 348818