swift234

[Sema] Honor -disable-availability-checking flag for protocol conformances

Suppress API availability diagnostics about protocol conformances when the
-disable-availability-checking frontend flag is passed. We weren't
checking the flag before, so there was no way to disable this diagnostic.

I've added a test to make sure this flag is honored. We can remove the test when
we remove the flag, which is temporary.

Swift SVN r27439