[WebAssembly] Add v128 value type

[tlively]

Summary

Depends on D52105.

[aheejin]

wasm/WriterUtils.cpp

189

Is there any reason why this is lowercase unlike others?

[tlively]

• Capitalize

wasm/WriterUtils.cpp

std::string lld::toString(ValType Type) {
   case ValType::I32:
     return "I32";
   case ValType::I64:
     return "I64";
   case ValType::F32:
     return "F32";
   case ValType::F64:
     return "F64";
+  case ValType::V128:
+    return "V128";
   case ValType::EXCEPT_REF:
     return "except_ref";
   }
   llvm_unreachable("Invalid wasm::ValType");
 }
 
 std::string lld::toString(const WasmSignature &Sig) {
   SmallString<128> S("(");