swift234

[SE-0112] Provide default implementations for CustomNSError requirements.

Provide default implementations for all of the CustomNSError requirements:
  * errorDomain: default to the name of the enum type
  * errorCode: default to the same thing "_code" gets, e.g., the enum tag or
    raw value
  * errorUserInfo: default to empty

This makes it significantly easier to customize just one aspect of the
NSError view of an error type, e.g., just the user-info dictionary,
without having to write boilerplate for the others. This was actually
part of SE-0112, but I missed it in the original implementation and we
thought it was an amendment.

Fixes rdar://problem/23511842.