Turned Sema::HandleDelayedAvailabilityCheck into a static function; NFC.

Did a bit of drive-by reformatting as well since it required rearranging some other static functions in the file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219795 91177308-0d34-0410-b5e6-96231b3b80d8