swift234

[Sema] Validate declaration version range availability within lexical scope

This commit validates @availability() attribute version ranges to ensure that
a declaration is not more available than its lexically containing scope. To do so,
we find the inner-most declaration containing an @availability attribute that itself
has an @availability attribute and check that first attribute's available
version range is contained in the enclosing declaration's available range. If not,
we emit a diagnostic.

This commit removes a FIXME for checking @availability and overrides. It appears that
the FIXME is a copy/paste to/from AttributeOverrideChecker, where it still resides.

Swift SVN r25453