libstdc++: Strip cv-qualifiers in std::atomic<FP> (PR 95282)

This squashes two commits, r11-674 and r11-1401.

	PR libstdc++/95282
	* include/bits/atomic_base.h (__atomic_impl::load): Add
	const-qualifier to parameter so that _Tp is deduced as the
	non-const type, and use _Val to get the unqualified type.
	(__atomic_impl::exchange): Use the _Val alias to remove
	volatile from the reinterpret_cast result type.
	* testsuite/29_atomics/atomic_float/95282.cc: New test.

(cherry picked from commit e40b11a91cb345db1324c3cb8f75b01e28056693)

libstdc++-v3/include/bits/atomic_base.h

@@ -850,21 +850,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { __atomic_store(__ptr, std::__addressof(__t), int(__m)); }
 
     template<typename _Tp>
-      _GLIBCXX_ALWAYS_INLINE _Tp
-      load(_Tp* __ptr, memory_order __m) noexcept
+      _GLIBCXX_ALWAYS_INLINE _Val<_Tp>
+      load(const _Tp* __ptr, memory_order __m) noexcept
       {
 	alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
-	_Tp* __dest = reinterpret_cast<_Tp*>(__buf);
+	auto* __dest = reinterpret_cast<_Val<_Tp>*>(__buf);
 	__atomic_load(__ptr, __dest, int(__m));
 	return *__dest;
       }
 
     template<typename _Tp>
-      _GLIBCXX_ALWAYS_INLINE _Tp
+      _GLIBCXX_ALWAYS_INLINE _Val<_Tp>
       exchange(_Tp* __ptr, _Val<_Tp> __desired, memory_order __m) noexcept
       {
         alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
-	_Tp* __dest = reinterpret_cast<_Tp*>(__buf);
+	auto* __dest = reinterpret_cast<_Val<_Tp>*>(__buf);
 	__atomic_exchange(__ptr, std::__addressof(__desired), __dest, int(__m));
 	return *__dest;
       }

libstdc++-v3/testsuite/29_atomics/atomic_float/95282.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <atomic>
+
+float
+test01()
+{
+  std::atomic<float> a;
+  return a.load();
+}
+
+float
+test02()
+{
+  volatile std::atomic<float> a;
+  return a.load();
+}