Despite their similar names and uses, these protocols no longer share
much functionality - the former is used to take @objc enums defined in
Swift that conform to Error and expose them as NSErrors, and the
latter handles NS_ERROR_ENUM C enums, which get imported into Swift as
a wrapper around NSError. We can actually simplify them quite a bit.
- Eliminate base protocol __BridgedNSError, which no longer provides
any implementation for _BridgedStoredNSError.
- Eliminate default implementations that match what the compiler would
synthesize.
- Adopt recursive constraints and where-clauses on associated types
(and update the Clang importer to handle this).
- Collapse signed and unsigned default implementations when reasonable.
- Fold _BridgedStoredNSError's _nsErrorDomain into the existing public
requirement CustomNSError.errorDomain.
rdar://problem/35230080
e202e90294