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

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Munesanz wrote:

> > This patch is now only for the front end. @shiltian @alexey-bataev, let us know what you think about landing just this. We are going to push another PR with the runtime changes, and that one will include numbers for performance.
> 
> Hi! Can someone post a link here to the PR for the OMP runtime changes? Thanks!

Hi! This PR initially contained both frontend and runtime changes. However the runtime changes were reverted for easier review. The PR for runtime is not created yet, but you can find all the initial changes (frontend+runtime) in the first commit of this PR:[] https://github.com/llvm/llvm-project/pull/66919/commits/5314fbb92feb078e36e0ec917fe87bd6dc05e12a)

We will now split this frontend PR in several ones, as @mikerice1969 suggested, and later create the runtime PR.

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