revert #75443, update mir validator
This PR reverts rust-lang#75443 to fix rust-lang#75992 and instead uses rust-lang#75419 to fix rust-lang#75313.
Adapts rust-lang#75419 to correctly deal with unevaluated constants as otherwise some `feature(const_evaluatable_checked)` tests would ICE.
Note that rust-lang#72793 was also fixed by rust-lang#75443, but as that issue only concerns `feature(type_alias_impl_trait)` I deleted that test case for now and would reopen that issue.
rust-lang#75443 may have also allowed some other code to now successfully compile which would make this revert a breaking change after 2 stable versions, but I hope that this is a purely theoretical concern.
See https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/generator.20upvars/near/214617274 for more reasoning about this.
r? `@nikomatsakis` `@eddyb` `@RalfJung`
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