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

[Shilei Tian via llvm-commits]

shiltian wrote:

It's fine that it will generate code that cannot be linked, as long as we don't have runtime use. We just need to check generated IR.

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