swift234

[Sema] Suppress warning about useless availability checks in playgrounds and script mode.

We normally report a warning when a #available() check will always be true
because of the minimum deployment target. These warnings are potentially
annoying when the developer either cannot change the minimum deployment target
from the default (as in playgrounds) or when doing so is burdensome (as for
command-line scripts, which would require passing a target triple) -- so
suppress them.

rdar://problem/21324005

Swift SVN r29582