A "permanently enabled" availability domain is one that has been declared
always available and is also simultaneously has either an attribute that
makes it deprecated or universally unavailable.
Emit fix-its that remove (or update) `@available` attributes that restrict
availability in a permanently enabled domain. Also, emit warnings about
`if #available` queries that always return true because they check a
permanently enabled domain.
Resolves rdar://157601761.
8a1338dfb2