Directly use ConstValue for single literals in blocks
Addresses the minimal repro in https://github.com/rust-lang/rust/issues/92186, but doesn't fix the underlying problem (which would be solved by solving the anon subst problem afaict).
I do, however, think that it makes sense in general to treat single literals in anon blocks as const values directly, especially in light of the problem that the issue refers to (anon const evaluation being postponed until infer variables in substs can be resolved, which was introduced by https://github.com/rust-lang/rust/pull/90023), i.e. while we do get warnings for those unnecessary braces, we should try to avoid errors caused by those braces if possible.
5a4f47460b