Metadata APIs need to be generalized to Value*

[maleadt]

LLVM currently defines them as:

int LLVMHasMetadata(LLVMValueRef Inst) {
  return unwrap<Instruction>(Inst)->hasMetadata();
}

LLVMValueRef LLVMGetMetadata(LLVMValueRef Inst, unsigned KindID) {
  auto *I = unwrap<Instruction>(Inst);
  assert(I && "Expected instruction");
  if (auto *MD = I->getMetadata(KindID))
    return wrap(MetadataAsValue::get(I->getContext(), MD));
  return nullptr;
}

This triggers an assertion with:

using LLVM

@dispose ctx=Context() mod=LLVM.Module("SomeModule") begin
    st = LLVM.StructType("SomeType")
    ft = LLVM.FunctionType(st, [st])
    fn = LLVM.Function(mod, "SomeFunction", ft)

    @show metadata(fn)
end

[maleadt]

Worked around in #418