Add separate AstInitialStatic node for static initializers

Static variable initializers run before initial blocks, so use an
explicitly different procedure type for them. This also enables us to
now raise errors for assignments to const variables in initial blocks.

src/V3Active.cpp

@@ -422,6 +422,14 @@ private:
     virtual void visit(AstActive* nodep) override {
         // Actives are being formed, so we can ignore any already made
     }
+    virtual void visit(AstInitialStatic* nodep) override {
+        // Relink to IACTIVE, unless already under it
+        UINFO(4, "    INITIAL " << nodep << endl);
+        const ActiveDlyVisitor dlyvisitor{nodep, ActiveDlyVisitor::CT_INITIAL};
+        AstActive* const wantactivep = m_namer.getIActive(nodep->fileline());
+        nodep->unlinkFrBack();
+        wantactivep->addStmtsp(nodep);
+    }
     virtual void visit(AstInitial* nodep) override {
         // Relink to IACTIVE, unless already under it
         UINFO(4, "    INITIAL " << nodep << endl);