[WebAssembly] Don't error on conflicting uses of prototype-less functions

When we can't determine with certainty the signature of a function
import we pick the fist signature we find rather than error'ing out.

The resulting program might not do what is expected since we might pick
the wrong signature.  However since undefined behavior in C to use the
same function with different signatures this seems better than refusing
to compile such programs.

Fixes PR40472

Differential Revision: https://reviews.llvm.org/D58304

llvm-svn: 354523