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

[Erich Keane via cfe-commits]

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@@ -262,12 +291,52 @@ OpenACCDirectiveKind ParseOpenACCDirectiveKind(Parser &P) {
   return DirKind;
 }
 
+bool ParseOpenACCClause(Parser &P) {
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erichkeane wrote:

So the individual clauses each have their own 'grammar'.  I DID comment the 'clause-list' below best I could, but there is no  clause 'grammar' in the standard.  The one's ive mentioned are the ones where it is all a single identifier-token (or 'auto' as you've seen :/).

I'll spend a few moments typing something up to make this more clear however.

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