swift234

Add availability checking for references to classes to Sema.

This commit adds availability checking for references to classes in Sema. We
now emit a diagnostic when a potentially unavailable class is referenced
in an expression, either directly (via a TypeExpr) or as the result of being
substituted as a replacement type in a call to a generic function. This
latter check is needed at the callsite because the callee may use its
formal type parameter to get at the metadata for the type.

This commit does not check declarations of variables, etc., with unavailable
types -- this will come in a later patch.


Swift SVN r22651