Rollup merge of #143096 - RalfJung:tag_for_variant, r=compiler-errors

tag_for_variant: properly pass TypingEnv

Hard-coding `fully_monomorphized` here does not seem right...

This came up [on Zulip](https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/.E2.9C.94.20VariantId.3DDiscriminant.20when.20tag.20is.20niche.20encoded.3F/with/526103956).

Author: matthiaskrgr