Fix Issue 21294 - Improve unhelpful error message with duplicate name… - #23821

…d argument in overload set

Named-argument errors (e.g. "parameter assigned twice") were only
reported via checkNamedArgErrorAndReportOverload() when the symbol
was an OverDeclaration (od). Plain function overloads declared in
the same scope use the fd.overnext chain instead, so the
hasOverloads branch in resolveFuncCall() never ran this check and
fell back to the generic "none of the overloads are callable"
message.

Call checkNamedArgErrorAndReportOverload(fd, ...) in that branch
too, since overloadApply() already supports walking fd.overnext.