[mlir] parseFunctionSignature doesn't report error for missing empty result type

If I call `call_interface_impl::parseFunctionSignature` with `mustParseEmptyResult` set to true, but the function call I'm parsing does not have an explicit empty return, the function returns failure without having reported an error. Other errors that this function can encounter are reported. I came across this via CIR, where we were expecting this function to report errors, so parsing silently failed when I hit a call without the explicit empty return.

Start by locating call_interface_impl::parseFunctionSignature and the existing tests for its parsing errors. Reproduce the mustParseEmptyResult case with a call lacking an explicit empty return, then verify that it reports an error instead of only returning failure.

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