Internals: More const. No functional change intended.

src/V3VariableOrder.cpp

@@ -78,7 +78,7 @@ unsigned VarTspSorter::s_serialNext = 0;
 class VariableOrder final {
     // NODE STATE
     //  AstVar::user1()    -> attributes, via m_attributes
-    AstUser1InUse m_user1InUse;  // AstVar
+    const AstUser1InUse m_user1InUse;  // AstVar
 
     struct VarAttributes {
         uint32_t stratum;  // Roughly equivalent to alignment requirement, to avoid padding