2023-08-15

Support coerced attrs value() and type

Summary:
You currently can't do anything with the `coerced_attr` except print them out. Let's implement the basic `type` and `value()` methods to work with coerced attrs, similar to how configured attrs has it.

Coerced attrs aren't really converted to `Value<'v>`s, which is why I think it's ok for this implementation to live in buck2_bxl crate, since that's the only place it would be used.

3 of the coerced attrs types (select, concat, and explicit configured dep) aren't well supported in this implementation, but let's at least support the majority of the coerced attrs for a first pass.

Reviewed By: scottcao

Differential Revision: D48323666

fbshipit-source-id: 16e93ed80b77d0d1804db5919547dd725e52d27a