Add HIP_CLANG_LAUNCHER for caching

[GZGavinZhao]

Fixes ROCm/llvm-project#72.

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]

Hello, thank you for your contribution! This project is in maintenance mode and is not accepting PRs. We are planning to archive this project next week.

Would you be able to move your PR to https://github.com/ROCm/llvm-project? hipcc is located in https://github.com/ROCm/llvm-project/tree/amd-staging/amd/hipcc (it should be the same).

[kzhuravl]

Hello, thank you for your contribution! This project is in maintenance mode and is not accepting PRs. We are planning to archive this project next week.

Would you be able to move your PR to https://github.com/ROCm/llvm-project? hipcc is located in https://github.com/ROCm/llvm-project/tree/amd-staging/amd/hipcc (it should be the same).

If not, let me know, I can help with that. Thanks again!

[GZGavinZhao]

No problem, moved to ROCm/llvm-project#14.