[Clang] Disable `libc` headers for offloading languages

These headers are currently broken when included from the offloading
languages like OpenMP, OpenCL, CUDA, and HIP. Turn this logic off so we
can compile these languages when the GPU libc is installed. I am
currently trying to remedy this and have made an RFC for it in libc,
see https://discourse.llvm.org/t/rfc-implementing-gpu-headers-in-the-llvm-c-library/71523.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D153578