rustc_const_eval::util contains some code that does not belong there

[RalfJung]

The rustc_const_eval::util module consists of functions that are never used inside rustc_const_eval, nor do they need any other part of this crate, so these are at the wrong location -- looks like a mistake that was introduced when rustc_mir was split up a while ago, and/or as part of the later split of rustc_mir_transform.

Cc @rust-lang/wg-const-eval @cjgillot
Any suggestions for where this code belongs instead? Some of these are exclusively used by rustc_mir_transform, so they could go there. Some are also used by rustc_borrowck, so I am not sure what the best location is for them. I just know rustc_const_eval is definitely wrong. :)

[cjgillot]

IIRC, I put these methods there because rustc_const_eval was the most downstream crate that both rustc_borrowck and rustc_mir_transform depended on. I know this is wrong, but I did not really know of a better place.