Internals: Refactor generate construct Ast handling (#6280) (#6470)

Internals: Refactor generate construct Ast handling (#6280)

We introduce AstNodeGen, the common base class of AstGenBlock,
AstGenCase, AstGenFor, and AstGenIf, which together represent all SV
generate constructs. Subsequently remove AstNodeFor, AstNodeCase
(AstCase is now directly derived from AstNodeStmt) and adjust internals
to work on the new representation.

Output is identical modulo hashes do to changed AstNode type ids, no
functional change intended.

Step towards #6280.

src/V3Delayed.cpp

@@ -1297,9 +1297,6 @@ class DelayedVisitor final : public VNVisitor {
         // Record write reference
         recordWriteRef(nodep, false);
     }
-    void visit(AstNodeFor* nodep) override {  // LCOV_EXCL_LINE
-        nodep->v3fatalSrc("For statements should have been converted to while statements");
-    }
     void visit(AstWhile* nodep) override {
         VL_RESTORER(m_inLoop);
         m_inLoop = true;