astgen: Enforce the use of aliased operand accessors (#4688)

This patch enforces the use of the most specific accessors for operands
which have an '@astgen alias' declaration, by making the superclass
accessors of the same operands private. This ensures client code is
cleaner as you can't use multiple different methods to reference the
same operands (which we used to in some places). Also prep for some
refactoring.

src/V3LinkParse.cpp

@@ -561,7 +561,7 @@ private:
             // Convert to AstSelLoopVars so V3LinkDot knows what's being defined
             AstNode* const newp
                 = new AstSelLoopVars{selp->fileline(), selp->fromp()->unlinkFrBack(),
-                                     selp->rhsp()->unlinkFrBackWithNext()};
+                                     selp->bitp()->unlinkFrBackWithNext()};
             selp->replaceWith(newp);
             VL_DO_DANGLING(selp->deleteTree(), selp);
         } else if (VN_IS(bracketp, SelLoopVars)) {