Introduce ns_error_domain attribute.

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 https://github.com/llvm/llvm-project-staging/commit/a14779f504b02ad0e4dbc39d6d10cadc7ed4cfd0
and adapted to updated Clang APIs.

Reviewed By: gribozavr2, aaron.ballman

Differential Revision: https://reviews.llvm.org/D84005