[WebAssembly] Make WasmSymbol's signature usable for events (NFC)

Summary:
WasmSignature used to use its `WasmSignature` member variable only for
function types, but now it also can be used for events as well.

Reviewers: sbc100

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

Differential Revision: https://reviews.llvm.org/D55247

llvm-svn: 348702