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

[via cfe-commits]

================
@@ -262,12 +291,52 @@ OpenACCDirectiveKind ParseOpenACCDirectiveKind(Parser &P) {
   return DirKind;
 }
 
+bool ParseOpenACCClause(Parser &P) {
----------------
cor3ntin wrote:

Can you add a description of the grammar?

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