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

[Jose Manuel Monsalve Diaz via cfe-commits]

josemonsalve2 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.

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