[WebAssembly] Error on mismatched function signature in final output

During symbol resolution, emit warnings for function signature
mismatches.  During GC, if any mismatched symbol is marked as live
then generate an error.

This means that we only error out if the mismatch is written to the
final output.  i.e. if we would generate an invalid wasm file.

Differential Revision: https://reviews.llvm.org/D48394

llvm-svn: 335192