[WebAssembly] Add equality comparison operators for WasmEventType

Summary:
This was missing in D54096. Independent tests for this is not available
here, because these are used in lld.

Reviewers: sbc100

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

Differential Revision: https://reviews.llvm.org/D54662

llvm-svn: 347154