[clang] [OpenACC] Add 'clause' parsing infrastructure plus a few clauses (PR #75052)

[Alexey Bataev via cfe-commits]

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@@ -69,6 +69,19 @@ enum class OpenACCAtomicKind {
   Capture,
   Invalid,
 };
+
+// Represents the kind of an OpenACC clause.
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alexey-bataev wrote:

Use '///' kind of comment here

https://github.com/llvm/llvm-project/pull/75052