Internal: Refactor AstAssignAlias (#6280) (#6473)

Rename AstAssignAlias to AstAlias and make it derive from AstNode
instead of AstNodeStmt.

Replace AstAlias with AstAssignW in V3LinkDot::linkDotScope, which is
the last place we need to be aware of the alias construct. Using
AstAssignW dowstream enables further optimization while preserving the
same functionality.

src/V3EmitV.cpp

@@ -155,7 +155,7 @@ class EmitVBaseVisitorConst VL_NOT_FINAL : public VNVisitorConst {
         iterateAndNextConstNull(nodep->rhsp());
         puts(";\n");
     }
-    void visit(AstAssignAlias* nodep) override {
+    void visit(AstAlias* nodep) override {
         putbs("alias ");
         iterateAndNextConstNull(nodep->lhsp());
         putfs(nodep, " = ");