Introduce CoercePointeeValidated for coherence checks at typeck stage

[dingxiangfei2009]

Fix #135206

This is the first PR to introduce the "wellformedness" check for derive(CoercePointee).

This patch introduces a new error code to cover all the prerequisites of the said macro. The checks that is enforced with this patch is whether the data is indeed struct and whether the layout is set to repr(transparent).

A following series of patch will arrive later to address the following concern.

1. CoercePointee leaks unstable unsizing impls #135217 so that we would only admit one single coercion on one type parameter, and leave the rest for future consideration in tandem of development of other coercion rules.
2. Enforcement of data field requirements.

An open question is whether there is a good schema to encode the #[pointee] as well, so that we could also check if the #[pointee] type parameter is indeed ?Sized.

@rustbot label F-derive_coerce_pointee