Auto merge of #120100 - oli-obk:astconv_lifetimes, r=BoxyUwU

Don't forget that the lifetime on hir types is `'tcx`

This PR just tracks the `'tcx` lifetime to wherever the original objects actually have that lifetime. This code is needed for rust-lang/rust#107606 (now #120131) so that `ast_ty_to_ty` can invoke `lit_to_const` on an argument passed to it. Currently the argument is `&hir::Ty<'_>`, but after this PR it is `&'tcx hir::Ty<'tcx>`.

Author: bors