The Clang attribute allows one to state that a particular enumeration
type describes an error, and associates it with a particular domain
constant. However, due to lack of API notes support, this attribute
wasn't actually getting used. Instead, we had a number of explicit
extensions to enum types to make them conform to the _BridgedNSError
protocol explicitly.
Now that we have API notes, use them to make these enums into error
enums with the appropriate domain, so that the Clang importer will
synthesize the _BridgedNSError conformances. Then, remove all of the
explicit conformances---and with them, the overlays for 12 frameworks.
There is a small fix to more eagerly consider these conformances as
"used" if an expression is formed with the error enum as a value
type. This better ensures that the conformances will be available at
runtime when needed.
This cleanup is needed to implement SE-0112 (NSError bridging),
although it is useful by itself.
