libexpr/attr-set.hh: Deprecate Attr default constructor

The default constructor of Attr default constructs a Value, which is
itself deprecated. Therefore the default constructor of Attr must
itself be either deprecated in turn or removed. The default can't be
trivially deleted because Bindings::EMPTY depends on the default
constructor of Bindings which depends on Attr's default constructor, so
I'm settling for deprecating it for now.

Part of work towards #744

Change-Id: Ie34b08788780615c5478a0354122530b6a6a6964

lix/libexpr/attr-set.hh

@@ -25,7 +25,8 @@ struct Attr
     PosIdx pos;
     mutable Value value;
     Attr(Symbol name, Value value, PosIdx pos = noPos) : name(name), pos(pos), value(value) {}
-    Attr() { };
+    [[deprecated]]
+    Attr() {};
     bool operator < (const Attr & a) const
     {
         return name < a.name;