Micro-cleanups around `StringPieceField`

* Make movable: no reason for it not to be, and this blocks some experiments.
* Fix compilation warnings.

PiperOrigin-RevId: 676689077

src/google/protobuf/string_piece_field_support_unittest.cc

@@ -0,0 +1,7 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2024 Google Inc.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+