swift234

[Sema] Remove 'deployment target ensures guard will always be true'

Remove the diagnostic that warns when an availability check is unnecessary
because the minimum deployment target ensures it will always be true. This
diagnostic is valuable (it tells users that they have dead fallback code) but
can also be super annoying when a source file is shared between projects with
different deployment targets. There is not currently a good mechanism to
suppress these warnings (for example, by expressing a "compatibility version" in
the source or as a build setting) and so it is better to turn the diagnostic
off.

rdar://problem/22337402