[clang] [Docs] Document freestanding requirements (PR #132232)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> This looks potentially-reasonable from the _Clang_ subproject POV, but from a whole-project POV, I think we ought to actually provide a conforming freestanding mode somehow, probably via llvm-libc.

+1

> So I'd kinda like to hear from llvm-libc folks what they're thinking here. @jhuber6 @michaelrj-google any thoughts?

My understanding (and correct me if I'm wrong!) is that llvm-libc aims to provide both a hosted and a freestanding mode.

https://github.com/llvm/llvm-project/pull/132232