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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

The post submit buildbots with the libc tag are all moved to newhdrgen now.  I'll wait for explicit acks from @petrhosek and @jhuber6 before attempting to crash-land this.

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