"constant expression depends on a generic parameter" error can be improved.

[eddyb]

Original discussion happened at #70452 (comment).

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This was my last proposal in that comment thread (#70452 (comment)):

So perhaps a better phrasing for the error itself might be "cannot prove/determine/guarantee/etc. that constant expression will evaluate successfully" and then we can search its Substs for type/const parameters and list them out to give the more helpful information.

We should also link https://github.com/rust-lang/rust/issues/68436, in some sort of help messaging along the lines of "there is no way to currently write the necessary where clause, watch this space".

cc @rust-lang/wg-diagnostics @varkor @yodaldevoid