Internal coding: Assert that user#() don't overlap.

Any use of a user() must now be declared, generally in the Visitor class with
    AstUser#InUse   arbitrary_object
This lets the code track if there's another request for the same user(),
preventing nasty hard to debug cases where they overlap.  This will also
call user#ClearTree(), so a bunch of those were removed, though many
extranious ones still remain.

src/V3Const.cpp

@@ -671,6 +671,7 @@ private:
 	else if (!m_cpp && nodep->lhsp()->castConcat()) {
 	    bool need_temp = false;
 	    if (m_warn && !nodep->castAssignDly()) {  // Is same var on LHS and RHS?
+		AstUserInUse	m_inuse1;
 		ConstVarMarkVisitor mark(nodep->lhsp());
 		ConstVarFindVisitor find(nodep->rhsp());
 		if (find.found()) need_temp = true;