FoundationEssentials: add `RawRepresentable` conformance to `_ErrorCo…

[compnerd]

…deProtocol`

The Self constraint suggests that the protocol provide a RawRepresentable conformance. Silence the compiler warning by adding the conformance.

[parkera]

I actually tried to fix this too, but it causes an ABI breakage on Darwin. I'm not sure it's possible to compatibly add a requirement to a protocol like this.

[compnerd]

@parkera oh, interesting. I wonder if we can add the conformance conditionally?

[parkera]

I think it boils down to a new requirement that would be imposed upon anyone who adopts this (outside the framework), which we don't have any mechanism to enforce. I could be wrong though.

[iCharlesHu]

@swift-ci please test macOS

[parkera]

@compnerd any idea what we can do about this one, given the ABI break?

[compnerd]

I need to try to build this again to remember what is going on here; I wonder if the constraint might be something that we need on the typealias? But either way, the new constraint is the issue. I'm not sure if there is a way to adopt this type without that, but at the same time, we don't have a way to silence the warning.