[CSBindings] Let key path subscript index be bound to existential in diagnostic mode

This parameter doesn't allow direct holes because it always replies
on the contextual type (if it's not present the hole should be
propagated to it), so if we know that the argument is some invalid
existential value (one without a superclass) let's allow binding to
it only in diagnostic mode which would be detected and diagnosed as
a contextual mismatch.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -1437,12 +1437,14 @@ PotentialBindings::inferFromRelational(Constraint *constraint) {
     // a valid key path type of its subtype.
     if (kind == AllowedBindingKind::Supertypes) {
       if (type->isExistentialType()) {
-        type = type->getExistentialLayout().explicitSuperclass;
+        auto layout = type->getExistentialLayout();
+        if (auto superclass = layout.explicitSuperclass) {
+          type = superclass;
+        } else if (!CS.shouldAttemptFixes()) {
+          return llvm::None;
+        }
       }
     }
-
-    if (!type)
-      return llvm::None;
   }
 
   if (auto *locator = TypeVar->getImpl().getLocator()) {