improve spans for `CallArgument` constraints

[lcnr]

that's a quirk of CallArgument constraints. they get their span from the Location of the call terminator; since they don't store the span of the particular argument, it gets lost in diagnostics. that said, it does look like we have the spans of each argument in TypeChecker::check_call_inputs, so they could store that.

Originally posted by @dianne in #133858 (comment)