Rollup merge of #118964 - compiler-errors:resolve, r=aliemjay

Opportunistically resolve region var in canonicalizer (instead of resolving root var)

See comment in `compiler/rustc_type_ir/src/infcx.rs`.

The **root** infer region for a given region vid may not actually be nameable from the universe of the original vid. That means that the assertion in the canonicalizer was too strict, since the `EagerResolver` that we use before canonicalizing is doing only as much resolving as it can.

This replaces `resolve_lt_var` and `probe_lt_var` in the `rustc_type_ir` API with `opportunistic_resolve_lt_var`, which acts as you expect it should. I left a FIXME that complains about the inconsistency.

This test is really gnarly, but I have no idea how to minimize it, since it seems to kind of just be coincidental that it triggered this issue. I hope the underlying root cause is easy enough to understand, though.

r? `@lcnr` or `@aliemjay`

Fixes #118950