Unavailable decls cannot be overridden by available decls. This change improves
the compiler diagnostics for this restriction in a few ways:
- Duplicate diagnostics are suppressed for getters and setters inside property
declarations that have already been diagnosed.
- Diagnostics are suppressed for implicit accessors since they would typically
duplicate diagnostics for the explicit accessors the implicit ones are
derived from.
- Decls inside unavailable a derived class are no longer incorrectly diagnosed.
