[clang] [OpenMP][CIR] Implement basic 'parallel' lowering + some clause infra (PR #172308)

[Erich Keane via cfe-commits]

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@@ -9527,7 +9527,9 @@ class OMPClauseVisitorBase {
 
 #define GEN_CLANG_CLAUSE_CLASS
 #define CLAUSE_CLASS(Enum, Str, Class)                                         \
-  RetTy Visit##Class(PTR(Class) S) { DISPATCH(Class); }
+  RetTy Visit##Class(PTR(Class) S) {                                           \
+    return static_cast<ImplClass *>(this)->VisitOMPClause(S);                  \
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erichkeane wrote:

Please note the consequences of this change.  This means you'll NOT be able to call `OMPClauseVisitorBase::VisitOMPIfClause`  (for example) directly.

However, this change DOES allow us to only partially implement a visitor (and have a 'fallback' option).  Else if you didn't implement a 'visit' clause, it ended up being an infinite loop, and `VisitOMPClause` was dead code.

This change doesn't seem to have broken anything, so I suspected that the 'feature' we're 'losing' was never  intended, and that this was the intended behavior.

https://github.com/llvm/llvm-project/pull/172308