FoundationEssentials on Darwin?

[taylorswift]

@Max_Desiatov

We already provide availability annotations for Darwin platforms and IIRC display those when rendered with DocC. I hope this could be extended to all platforms, not only Darwin.

IIRC, lib/SymbolGraphGen does not pick up on platform-gated API across #if boundaries, it always builds for the host platform and skips everything else.