[CSBindings] Mark generic parameter type vars as complete if they don't have any adjacent vars

Always marking generic parameter type variables as incomplete
is too aggressive. That was the way to make sure that they
are never attempted to eagerly, but really there are only a
couple of situations where that can cause issues:

```
1. Int <: $T_param
   $T1 <: $T_param

2. $T2 conv Generic<$T_param>
   $T2 conv Generic<Int?>
   Int <: $T_param
```

Attempting $T_param before $T1 in 1. could result in a missed
optional type binding for example.

Attempting $T_param too early in this case (before $T2) could
miss some transitive bindings inferred through conversion
of `Generic` type.

If a type variable that represents a generic parameter is no longer
associated with any type variables (directly or indirectly) it's safe
to assume that its binding set is complete.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -173,9 +173,30 @@ bool BindingSet::involvesTypeVariables() const {
 }
 
 bool BindingSet::isPotentiallyIncomplete() const {
-  // Generic parameters are always potentially incomplete.
+  // Always marking generic parameter type variables as incomplete
+  // is too aggressive. That was the way to make sure that they
+  // are never attempted to eagerly, but really there are only a
+  // couple of situations where that can cause issues:
+  //
+  // 1. Int <: $T_param
+  //    $T1 <: $T_param
+  //
+  // 2. $T2 conv Generic<$T_param>
+  //    $T2 conv Generic<Int?>
+  //    Int <: $T_param
+  //
+  // Attempting $T_param before $T1 in 1. could result in a missed
+  // optional type binding for example.
+  //
+  // Attempting $T_param too early in this case (before $T2) could
+  // miss some transitive bindings inferred through conversion
+  // of `Generic` type.
+  //
+  // If a type variable that represents a generic parameter is no longer
+  // associated with any type variables (directly or indirectly) it's safe
+  // to assume that its binding set is complete.
   if (Info.isGenericParameter())
-    return true;
+    return involvesTypeVariables();
 
   // Key path literal type is incomplete until there is a
   // contextual type or key path is resolved enough to infer