Auto merge of #109119 - lcnr:trait-system-cleanup, r=compiler-errors

a general type system cleanup

removes the helper functions `traits::fully_solve_X` as they add more complexity then they are worth. It's confusing which of these helpers should be used in which context.

changes the way we deal with overflow to always add depth in `evaluate_predicates_recursively`. It may make sense to actually fully transition to not have `recursion_depth` on obligations but that's probably a bit too much for this PR.

also removes some other small - and imo unnecessary - helpers.

r? types

Author: bors