[WebAssembly] Make sret parameter work with AddMissingPrototypes

Summary:
Even with functions with `no-prototype` attribute, there can be an
argument `sret` (structure return) attribute, which is an optimization
when a function return type is a struct. Fixes PR42420.

Reviewers: sbc100

Subscribers: dschuff, jgravelle-google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64318

llvm-svn: 365426