Sema: Use ASSERT before getting trailing objects of Constraint

If the precondition doesn't hold, we will return a pointer to
some random memory, so it's best to always crash since this
indicates something is seriously wrong.

Author: slavapestov

include/swift/Sema/Constraint.h

@@ -872,7 +872,7 @@ class Constraint final : public llvm::ilist_node<Constraint>,
 
   /// Retrieve the overload choice for an overload-binding constraint.
   OverloadChoice getOverloadChoice() const {
-    assert(Kind == ConstraintKind::BindOverload);
+    ASSERT(Kind == ConstraintKind::BindOverload);
     return *getTrailingObjects<OverloadChoice>();
   }
 
@@ -903,7 +903,7 @@ class Constraint final : public llvm::ilist_node<Constraint>,
 
   /// Retrieve the DC in which the overload was used.
   DeclContext *getDeclContext() const {
-    assert(HasDeclContext);
+    ASSERT(HasDeclContext);
     return *getTrailingObjects<DeclContext *>();
   }