[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

[ajarmusch]

@erichkeane I added builtin support for -fexperimental-openacc-macro-override, I also added support for a failed list if we wanted to use it. Similar to how the SOLLVE project does it

[erichkeane]

@erichkeane I added builtin support for -fexperimental-openacc-macro-override, I also added support for a failed list if we wanted to use it. Similar to how the SOLLVE project does it

Neat, that will be really handy! Unfortunately I have no experience modifying stuff here, but @omjavaid and @tarunprabhu "touched it last", so I'm hopeful they can take a look.

[ajarmusch]

Thanks @erichkeane, Anyone have any comments for this PR?