fix c++20-compat false positive (#909)

* fix c++20-compat false positive

* move the c++20 compat warning suppression to a separate header

* fix GCC/clang in private/diagnostic_cxx_20_compat_push.h

---------

Co-authored-by: Jan Dorniak <[email protected]>

include/etl/platform.h

@@ -358,9 +358,11 @@ SOFTWARE.
 //*************************************
 // Determine if the ETL can use char8_t type.
 #if ETL_NO_SMALL_CHAR_SUPPORT
+#include "private/diagnostic_cxx_20_compat_push.h"
   typedef uint_least8_t char8_t;
   #define ETL_HAS_CHAR8_T 1
   #define ETL_HAS_NATIVE_CHAR8_T 0
+#include "private/diagnostic_pop.h"
 #else
   #define ETL_HAS_CHAR8_T 1
   #define ETL_HAS_NATIVE_CHAR8_T 1

include/etl/private/diagnostic_cxx_20_compat_push.h

@@ -0,0 +1,44 @@
+///\file
+
+/******************************************************************************
+The MIT License(MIT)
+
+Embedded Template Library.
+https://github.com/ETLCPP/etl
+https://www.etlcpp.com
+
+Copyright(c) 2023 John Wellbelove
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files(the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and / or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions :
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+******************************************************************************/
+
+/*
+ * The header include guard has been intentionally omitted.
+ * This file is intended to evaluated multiple times by design.
+ */
+
+#if defined(__GNUC__) && !defined(__clang__) && !defined(__llvm__)
+  #pragma GCC diagnostic push 
+  #pragma GCC diagnostic ignored "-Wc++20-compat"
+#endif
+
+#if defined(__clang__) || defined(__llvm__)
+  #pragma clang diagnostic push 
+  #pragma clang diagnostic ignored "-Wc++20-compat"
+#endif