[llvm] [runtimes] Make later runtimes wait for earlier header targets (PR #192814)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 commented:

`flang-rt` can depend on `libc++` headers. Do you know if they do anything special for this? At a first look this seems a little verbose for what should just be a standard dependency.

Thinking back on this, I think libc++ and flang-rt handle this by hooking up to the libc target directly under some condition, for the GPU we could just do this directly since LLVM libc is the only game in town for C headers  there. Let me know if that makes sense.

https://github.com/llvm/llvm-project/pull/192814