rustdoc: Consider adding CCI for the HIR expressions of public constants

[camelid]

When a constant is inlined into another crate, we are forced to always show the evaluated/folded version of the constant. This is because we don't have access to the original HIR in the metadata.

This could be a situation where we can use (rust-lang/rfcs#3662) to store the original expression for public constants (that could potentially be inlined by other crates) so that we don't have the cross-crate issue. Might be applicable to other cross-crate inconsistencies too (though hopefully the type alias one will be resolved by lazy_type_alias).

Adapted from a post by @camelid in #131975 (comment)