Auto merge of #25065 - quantheory:fix_associated_const_ambiguity_message, r=nikomatsakis

This fixes #24922 and #25017, and reduces the number of error messages that talk about "methods" when associated constants rather than methods are involved.

I will admit that I haven't thought very carefully about the error messages. My goal has been to make more of the messages technically correct in all situations, and to avoid ICEs. But in some cases we could probably talk specifically about "methods" rather than "items".