misc coercion cleanups and handle safety correctly

[BoxyUwU]

r? lcnr

"remove normalize call"

Fixes #132765

If the normalization fails we would sometimes get a TypeError containing inference variables created inside of the probe used by coercion. These would then get leaked out causing ICEs in diagnostics logic

"leak check and lub for closure<->closure coerce-lubs of same defids"

Fixes rust-lang/trait-system-refactor-initiative#233

fn peculiar() -> impl Fn(u8) -> u8 {
    return |x| x + 1
}

the |x| x + 1 expr has a type of Closure(?31t) which we wind up inferring the RPIT to. The CoerceMany ret_coercion for the whole peculiar typeck has an expected type of RPIT (unnormalized). When we type check the return |x| x + 1 expr we go from the never type to Closure(?31t) which then participates in the ret_coercion giving us a coerce-lub(RPIT, Closure(?31t)).

Normalizing RPIT gives us some Closure(?50t) where ?31t and ?50t have been unified with ?31t as the root var. resolve_vars_if_possible doesn't resolve infer vars to their roots so these wind up with different structural identities so the fast path doesn't apply and we fall back to coercing to a fn ptr. cc #147193 which also fixes this

New solver probably just gets more inference variables here because canonicalization + generally different approach to normalization of opaques. Idk :3

FCP worthy stuffy

there are some other FCP worthy things but they're in my FCP comment which also contains some analysis of the breaking nature of the previously listed changes in this PR: #148602 (comment)