ns_error_domain can be used by, e.g. NS_ERROR_ENUM, in order to
identify a global declaration representing the domain constant.
Introduces the attribute, Sema handling, diagnostics, and test case.
This is cherry-picked from a14779f504
and adapted to updated Clang APIs.
Reviewed By: gribozavr2, aaron.ballman
Differential Revision: https://reviews.llvm.org/D84005