[clang] [OpenMP] Introduce support for OMPX extensions and taskgraph frontend (PR #66919)

[Jose Manuel Monsalve Diaz via cfe-commits]

josemonsalve2 wrote:

> Waiting for the evaluation, if possible

Thanks Shilai, Chenle is working on the evaluation. 

The one aspect that we are still not sure what to do about is if we should split this up into multiple PRs or not. 

https://github.com/llvm/llvm-project/pull/66919