[clang] [HIP] Add libhipcxx to the default header search path (PR #214279)

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 commented:

Once upon a  time I was looking into building `libc++` headers with implicit declare all on the device and linking in `libc++.a` for GPUs. The libhipcxx thing is basically an ad-hoc wrapping of libstdc++ within a custom namespace, right?

https://github.com/llvm/llvm-project/pull/214279