Investigate why named arguments in FormatSpec.precision seem to have indices that don't correspond to a span in Context.arg_spans

[PrestonFrom]

As noted int #99263, named arguments in FormatSpec.precision seem to have indices that don't correspond to a span in Context.arg_spans. This caused an ICE with the named_arguments_used_positionally lint, which assumed the index in the FxHashSet for named arguments would correspond to the index for arg_spans in context.

Determine if this is a bug or if some sort of translation is necessary and update accordingly.

[PrestonFrom]

@rustbot claim