Fix uses of __cpp_lib_constexpr_string (#18890)

The intent of these checks was that if the C++20 version of this feature
are enabled to execute the top codepath. libstdc++ 12 defines this
feature as:

```cpp
// in: /usr/include/c++/12/bits/basic_string.h

#ifdef __cpp_lib_is_constant_evaluated
// Support P0980R1 in C++20.
# define __cpp_lib_constexpr_string 201907L
#elif __cplusplus >= 201703L && _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED
// Support P0426R1 changes to char_traits in C++17.
# define __cpp_lib_constexpr_string 201611L
#endif
```

So this codepath was always being hit even with -std=c++17 and then
resulted in a failure because the string wasn't actually constinit. This
matches the other use of this feature check in `inlined_string_field.h`

Closes #18890

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/18890 from keith:ks/fix-uses-of-__cpp_lib_constexpr_string edb4f5f6b6818496e56706a58b6db21315a83c22
PiperOrigin-RevId: 696526963

csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs

diff --git a/csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs b/csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs
deleted file mode 100644
index 208ce1f..0000000
--- a/csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs
+++ /dev/null
@@ -1,17 +0,0 @@
-#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2008 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
-#endregion
-
-namespace Google.Protobuf.Reflection;
-
-internal sealed partial class FeatureSetDescriptor
-{
-    // Canonical serialized form of the edition defaults, generated by embed_edition_defaults.
-    private const string DefaultsBase64 =
-        "ChMYhAciACoMCAEQAhgCIAMoATACChMY5wciACoMCAIQARgBIAIoATABChMY6AciDAgBEAEYASACKAEwASoAIOYHKOgH";
-}

src/google/protobuf/port.cc

diff --git a/src/google/protobuf/port.cc b/src/google/protobuf/port.cc
index 0261a94..af668e9 100644
--- a/src/google/protobuf/port.cc
+++ b/src/google/protobuf/port.cc
@@ -97,7 +97,7 @@
   }
 }
 
-#if defined(__cpp_lib_constexpr_string)
+#if defined(__cpp_lib_constexpr_string) && __cpp_lib_constexpr_string >= 201907L
 PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT const GlobalEmptyString
     fixed_address_empty_string{};
 #else

src/google/protobuf/port.h

diff --git a/src/google/protobuf/port.h b/src/google/protobuf/port.h
index ec00308..8931d43 100644
--- a/src/google/protobuf/port.h
+++ b/src/google/protobuf/port.h
@@ -454,7 +454,7 @@
 // Default empty string object. Don't use this directly. Instead, call
 // GetEmptyString() to get the reference. This empty string is aligned with a
 // minimum alignment of 8 bytes to match the requirement of ArenaStringPtr.
-#if defined(__cpp_lib_constexpr_string)
+#if defined(__cpp_lib_constexpr_string) && __cpp_lib_constexpr_string >= 201907L
 // Take advantage of C++20 constexpr support in std::string.
 class alignas(8) GlobalEmptyString {
  public: