[libc++][NFC] Refactor _LIBCPP_AVAILABILITY_HAS_* macros to always be defined (#71002)

This makes the conditionals quite a bit simpler to understand, since it
avoids double negatives and makes sure we have <__availability>
included. For vendors which use availability macros, it also enforces
that they check when specific features are introduced and define the
macro for their platform appropriately.