asm-llvm: improve error message for duplicate bindings.
asm-llvm: Update call-site coercions to match latest spec proposal.

This matches https://bugzilla.mozilla.org/show_bug.cgi?id=883175#c7,
except that we still have a 'maybevoid' type.  (I'll get rid of that
in the next commit.)
asm-llvm: Remove MaybeVoid type.

Functions which return double or int must always have the proper coercion.