Add HIP_CLANG_LAUNCHER for launching Clang through a wrapper executable

[GZGavinZhao]

Fixes ROCm/HIPCC#147. Moved from ROCm/HIPCC#148.

This allows Clang to be called through a compiler wrapper/launcher. This is most commonly used to run clang through a compiler launcher that can cache compilations, such as ccache and sccache, to allow caching of compilation artifacts.

For a more detailed motivation and description, please see ROCm/ROCm#2817.

[kzhuravl]

@david-salinas, please review this. Thanks!