[External] Add OpenACC Validation and Verification Testsuite

[ajarmusch]

In support of the recent ongoing efforts to implement OpenACC in Clang/LLVM, we want to introduce the External V&V Testsuite.

The OpenACC Validation and Verification Team at the University of Delaware (UD), implements an independent V&V testsuite, to verify OpenACC implementations across various compilers and system architectures.

Part of this PR uses a module from CMake called FindOpenACC, I have created a PR to add support for the Clang Compiler within CMake. - FindOpenACC: Add support for the Clang Compiler

Continuation of #38 by @wangxin0321