Revise the "too complex" heuristic.

Revise the heuristic so that it does not differ between Swift
versions, and so that it is always based on the number of scopes we've
opened.

The memory threshold and (very long) timer are also still in place as
well.

Author: rudkx

lib/Sema/ConstraintSystem.h

@@ -3136,26 +3136,9 @@ class ConstraintSystem {
       return true;
     }
 
-    if (!getASTContext().isSwiftVersion3()) {
-      if (CountScopes < TypeCounter)
-        return false;
-
-      // If we haven't explored a relatively large number of possibilities
-      // yet, continue.
-      if (CountScopes <= 16 * 1024)
-        return false;
-
-      // Clearly exponential
-      if (TypeCounter < 32 && CountScopes > (1U << TypeCounter))
-        return true;
-
-      // Bail out once we've looked at a really large number of
-      // choices.
-      if (CountScopes > TC.Context.LangOpts.SolverBindingThreshold)
-        return true;
-    }
-
-    return false;
+    // Bail out once we've looked at a really large number of
+    // choices.
+    return CountScopes > TC.Context.LangOpts.SolverBindingThreshold;
   }
 
   LLVM_ATTRIBUTE_DEPRECATED(