AST: Turn NominalTypeDecl::getStoredProperties() into a request

This improves on the previous situation:

- The request ensures that the backing storage for lazy properties
  and property wrappers gets synthesized first; previously it was
  only somewhat guaranteed by callers.

- Instead of returning a range this just returns an ArrayRef,
  which simplifies clients.

- Indexing into the ArrayRef is O(1), which addresses some FIXMEs
  in the SIL optimizer.

lib/SILGen/SILGenFunction.cpp

@@ -803,9 +803,8 @@ SILValue SILGenFunction::emitUnwrapIntegerResult(SILLocation loc,
   while (!value->getType().is<BuiltinIntegerType>()) {
     auto structDecl = value->getType().getStructOrBoundGenericStruct();
     assert(structDecl && "value for error result wasn't of struct type!");
-    assert(std::next(structDecl->getStoredProperties().begin())
-           == structDecl->getStoredProperties().end());
-    auto property = *structDecl->getStoredProperties().begin();
+    assert(structDecl->getStoredProperties().size() == 1);
+    auto property = structDecl->getStoredProperties()[0];
     value = B.createStructExtract(loc, value, property);
   }