Rollup merge of #135195 - oli-obk:push-toyoyrupruko, r=lcnr

Make `lit_to_mir_constant` and `lit_to_const` infallible

My motivation for this change is just that it's annoying to check everywhere, especially since all but one call site was just ICEing on errors anyway right there.

They can still fail, but now just return an error constant instead of having the caller handle the error.

fixes #114317
fixes #126182

Author: matthiaskrgr