Rollup merge of #143235 - compiler-errors:const-item-bound, r=oli-obk

Assemble const bounds via normal item bounds in old solver too

Fixes the first example in https://rust-lang.zulipchat.com/#narrow/channel/144729-t-types/topic/elaboration.20of.20const.20bounds.3F/with/526378135

The code duplication here is not that nice, but it's at least very localized.

cc `@davidtwco`

r? oli-obk

Author: matthiaskrgr