swift234

[Clang Importer] Import ns_error_domain attribute with _BridgedNSError

ns_error_domain can now be used to communicate with the ClangImporter
when an enum has an associated error domain string. In this case, when
we import it as a Swift enum, we can also synthesize a conformance to
_BridgedNSError.

This allows the creation of something like NS_ERROR_ENUM, in which the
developer can declare an enum for the purposes of error handling. Adds
Sema and executable tests demonstrating this funcionality.

In order for the imported ns_error_domain bridging to work, we have to
at some point forcibly pull in a _BridgedNSError conformance, as one
will not be pulled in normally. This is a problem, and is explicitly
signaled in the provided test case