[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348702 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed