[Macros] Don't apply member attribute macros to accessors
Accessors are conceptually nested within their property or subscript
declarations, so member-attribute macros should only apply to the
property or subscript declarations. This eliminates what appears to be
redundant macro expansion.
Fixes rdar://118956800.
(cherry picked from commit 3f518a3b7932367b78fe7d9f2f0e1b2fbb086f78)
