[RFC] Adding support for Clang within the OpenACC module

Hello,

In support of the recent ongoing efforts to implement OpenACC in Clang/LLVM, Given LLVM’s utilization of CMake, we want to add support for the LLVM/Clang compiler in the OpenACC module.

This would be highly used, especially when we include the OpenACC Testsuite to the llvm-test-suite, which also uses CMake.

Similar to the compilers already supported, we see the only addition to the CMake code is within the FindOpenACC.cmake file, where we set ACC_FLAG_Clang.

Comments are welcome!

Thanks,
Aaron

@ajarmusch yes, one should only need to fill out the table in FindOpenACC. Please see the contribution guide. Thanks!