Avoid nullptr dereferencing of 'Constraint'; NFC

Klocwork static code analysis exposed this bug:
Pointer 'Constraint' returned from call to function
'cast_or_null<clang::ConceptSpecializationExpr,clang::Expr>' may
be NULL and will be dereferenced in the statement following it

Replace 'cast_or_null' with 'cast' so that the latter can assert
when it encounters a NULL.

This also removes an 'assert' that is covered by the previous
call to 'cast<>'.

clang/lib/Sema/SemaConcept.cpp

@@ -1062,11 +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_or_null<ConceptSpecializationExpr>(
-          TC->getImmediatelyDeclaredConstraint());
+      cast<ConceptSpecializationExpr>(TC->getImmediatelyDeclaredConstraint());
   bool Dependent =
       Constraint->getTemplateArgsAsWritten() &&
       TemplateSpecializationType::anyInstantiationDependentTemplateArguments(