swift234

Add type checking for potentially unavailable method references.

This commit modifies Sema to add type checking for potentially unavailable
method references. We now record the reason for method unavailability when
recording a potential overload choice during member constraint simplification
and either diagnose or lift to an optional type during CSApply. This commit also
generalizes UnavailableToOptionalExpr to take an arbitrary subexpression.

This commit does not address potentially unavailable properties, initializers,
or dynamic member references.


Swift SVN r22508