[compiler-rt] [llvm] [openmp] [Runtimes] Default build must use its own output dirs (PR #168266)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> Yeah, do you think we should move that to LLVM as a tool and pass in a path to the runtime build? Makes it slightly more annoying.

That is one way, but also consider that libc moved away from this because of this being an ["awkward dependency on the rest of LLVM"](https://discourse.llvm.org/t/rfc-new-headergen-proposal/79679). Now it is written in [Python](https://github.com/llvm/llvm-project/tree/main/libc/utils/hdrgen).

https://github.com/llvm/llvm-project/pull/168266