[WebAssembly] Fix local references to weak aliases

When weak aliases are used with in same translation
unit we need to be able to directly reference to alias
and not just the thing it is aliases.  We do this by
defining both a wasm import and a wasm export in this
case that result in a single Symbol.  This change is
a partial revert of rL314245.  A corresponding lld
change address the previous issues we had with this.

See: https://github.com/WebAssembly/tool-conventions/issues/34

Differential Revision: https://reviews.llvm.org/D41472

llvm-svn: 321242