Sema: Treat 'strict' availability flag like unavailable

This is a follow-up to r261512, which made the 'strict' availability
attribute flag behave like 'unavailable'.  However, that fix was
insufficient.  The following case would (erroneously) error when the
deployment target was older than 10.9:

    struct __attribute__((availability(macosx,strict,introduced=10.9))) A;
    __attribute__((availability(macosx,strict,introduced=10.9))) void f(A*);

The use of A* in the argument list for f is valid here, since f and A
have the same availability.

The fix is to return AR_Unavailable from DeclBase::getAvailability
instead of AR_NotYetIntroduced.  This also reverts the special handling
added in r261163, instead relying on the well-tested logic for
AR_Unavailable.

rdar://problem/23791325

llvm-svn: 262915