[darwin] add support for __isPlatformVersionAtLeast check for if (@available)

The __isPlatformVersionAtLeast routine is an implementation of `if (@available)` check
that uses the _availability_version_check API on Darwin that's supported on
macOS 10.15, iOS 13, tvOS 13 and watchOS 6.

Differential Revision: https://reviews.llvm.org/D90367