Adding an assertion back.

This assert was removed in 98339f14a0,
but during post-commit review, it was pointed out that the assert was
valid.

clang/lib/Sema/SemaConcept.cpp

@@ -1062,6 +1062,8 @@ ReturnTypeRequirement(TemplateParameterList *TPL) :
   assert(TPL->size() == 1);
   const TypeConstraint *TC =
       cast<TemplateTypeParmDecl>(TPL->getParam(0))->getTypeConstraint();
+  assert(TC &&
+         "TPL must have a template type parameter with a type constraint");
   auto *Constraint =
       cast<ConceptSpecializationExpr>(TC->getImmediatelyDeclaredConstraint());
   bool Dependent =