Auto merge of #124187 - compiler-errors:self-ctor, r=petrochenkov

Warn (or error) when `Self` ctor from outer item is referenced in inner nested item

This implements a warning `SELF_CONSTRUCTOR_FROM_OUTER_ITEM` when a self constructor from an outer impl is referenced in an inner nested item. This is a proper fix mentioned rust-lang/rust#117246 (comment).

This warning is additionally bumped to a hard error when the self type references generic parameters, since it's almost always going to ICE, and is basically *never* correct to do.

This also reverts part of rust-lang/rust#117246, since I believe this is the proper fix and we shouldn't need the helper functions (`opt_param_at`/`opt_type_param`) any longer, since they shouldn't really ever be used in cases where we don't have this problem.

Author: bors