[WebAssembly] Default to something reasonable in WebAssemblyAddMissingPrototypes

Previously if we couldn't derive a prototype for a "no-prototype"
function from C we would leave it as is:

  void foo(...)

With this change we instead give is an empty signature and remove
the "no-prototype" attribute.

This fixes the current wasm waterfall test failure.

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58488

llvm-svn: 354544