Because we don't form a type-checked call to the Regex initializer in the AST, we need to explicitly handle the availability checking for Regex<Output> and the initializer we're implicitly calling.
rdar://92156542
[Sema] Diagnose regex literals if `Regex<Output>` is unavailable
5.7 cherry-pick of #42573
Because we don't form a type-checked call to the Regex initializer in the AST, we need to explicitly handle the availability checking for
Regex<Output>
and the initializer we're implicitly calling.rdar://92156542