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
...and bump our "Foundation epoch" guard to 3 to match.
Long-term we should switch to an attribute for "yes, this is really a set
of enumerated constants" (and "this is a set of related bitmask options"),
but this unblocks things for now, and doesn't cause any issue if we end
up doing something else.
rdar://problem/20418505
Swift SVN r26944
