[WebAssembly] Use wasm::Signature for in ObjectWriter (NFC)

There are two `WasmSignature` structs, one in
include/llvm/BinaryFormat/Wasm.h and the other in
lib/MC/WasmObjectWriter.cpp. I don't know why they got separated in this
way in the first place, but it seems we can unify them to use the one in
Wasm.h for all cases.

Reviewed By: dschuff, sbc100

Differential Revision: https://reviews.llvm.org/D88428