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

[Alexey Bataev via cfe-commits]

================
@@ -2729,6 +2729,55 @@ class OMPTaskwaitDirective : public OMPExecutableDirective {
   }
 };
 
+/// This represents '#pragma ompx taskgraph' directive.
+/// Available with OMPX extensions.
+///
+/// \code
+/// #pragma ompx taskgraph
+/// \endcode
+///
+class OMPTaskgraphDirective : public OMPExecutableDirective {
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alexey-bataev wrote:

```
class OMPTaskgraphDirective final : public OMPExecutableDirective {
```

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