Symbolic solver: never raise Early_Binding_Error exceptions

The very way the symbolic solver works (topological sorting, often
on partial lists of atoms, stopping atoms collections early) prevents
most opportunities to detect equations that are malformed wrt. data
flow.

To avoid API predictability, just never raise Early_Binding_Error
exceptions and always consider a missing variable definition as a
"false" solution.

TN: SB20-024

Author: pmderodat

support/langkit_support-adalog-symbolic_solver.adb

@@ -1903,24 +1903,17 @@ package body Langkit_Support.Adalog.Symbolic_Solver is
 
       Ret : Boolean;
    begin
-      --  If the logic variable that ``Self`` uses is not defined, raise an
-      --  error.
+      --  Do not even try to evaluate this atom if it uses a variable that is
+      --  not defined at this point.
       --
       --  Note that this cannot happen when called from ``Solve_Compound`` as
       --  the topological sort makes sure all variables are defined before they
       --  are used (and abort the resolution if it is not possible), so the
       --  condition below will succeed only when ``Solve_Atomic`` is called
       --  from ``Solve`` when called on an atom directly.
-      --
-      --  TODO??? Maybe we should always go through ``Solve_Compound`` to avoid
-      --  this redundant check, and more generally have a unique way to solve
-      --  relations, and unique way to deal with errors (return no solution
-      --  or raise ``Early_Binding_Error``.
+
       if not Is_Defined_Or_Null (Used_Var (Atom)) then
-         raise Early_Binding_Error with
-           "Relation " & Image (Atom)
-           & " needs var " & Image (Used_Var (Atom).Logic_Var)
-           & " to be defined";
+         return False;
       end if;
 
       case Atom.Kind is