[sema] Fix availability narrowing fixits for macOS >= 11
For macOS >= 11 the fixit for narrowing an availability check rather
than adding a new one should fire if the major version is the same, like
on other platforms.
(cherry picked from commit c957826c286a5db3fe127eb6aa068371f38896cd)
(cherry picked from commit 643197d200bdb908163f400ca8ec20b162908fb5)
