swift234

Sema: Check getter and setter availability.

This commit adds checking for accesses of potentially unavailable getters and setters.
When walking an expression to check availability, AvailabilityWalker now keeps track of
the context for when it encounters a MemberRefExpr. This context be either
(1) the next encountered member reference could cause  the member's getter to be called
(e.g., the member ref is for a read of the property); (2) the member's setter could be
called (e.g., the member ref is the left-hand-side of an assignment);  or (3) the member
ref generates the lvalue for for an InOutExpr (&) -- in which case we have to assume both
the getter and the setter could be called. These diagnostics are protected by the
-enable-experimental-availability-checking flag.

This commit does not import separate getter and setter availability from Objective-C; that
is coming in a future commit.

Swift SVN r24870