Auto merge of #144377 - camsteffen:simplify-impl-of-method, r=fee1-dead

Rename impl_of_method and trait_of_item

This PR used to tweak the implementation of impl_of_method, but that introduced a perf regression.

Rename impl_of_method and trait_of_item to impl_of_assoc and trait_of_assoc respectively. This reflects how the two functions are closely related. And it reflects the behavior more accurately as the functions check whether the input is an associated item.