[libc] [llvm] [libc] delete hdrgen (PR #117220)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

rebasing on top of https://github.com/llvm/llvm-project/commit/ce416a073be6da06be7fd2d427b896b380fb3877 for the presubmit bots...

/me holds breath
/me dies

https://github.com/llvm/llvm-project/pull/117220