[clang] [OpenACC] Implement 'if' clause for Compute Constructs (PR #88411)

[Erich Keane via cfe-commits]

erichkeane wrote:

> General question. Should there be some support in clang/tools/libclang/CIndex.cpp?

Sorry, I only just saw this!  I guess I don't have a good idea what the CIndex stuff does.  I see there is quite a bit of this 'Enqueue' functionality in CIndex.cpp, but it isn't clear what it should be doing or what I should be doing here.  

https://github.com/llvm/llvm-project/pull/88411