libstdc++: Ignore cv-quals when std::allocator<void> constructs

When I added the std::allocator_traits<std::allocator<void>>
specialization it broke code like this:

  std::allocate_shared<const int>(std::allocator<void>());

The problem is that allocator_traits<allocator<void>>::construct(a, p)
now uses std::_Construct(p), which only does a static_cast<void*>(p) and
so fails if the pointer has cv-quals.

This changes std::_Construct (and the related std::_Construct_novalue)
to use a C-style cast to (void*) which matches the effects of the
"voidify" helper in the C++20 standard.

libstdc++-v3/ChangeLog:

	* include/bits/stl_construct.h (_Construct, _Construct_novalue):
	Also cast away cv-qualifiers when converting pointer to void.
	* testsuite/20_util/allocator/void.cc: Test construct function
	with cv-qualified types.

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

@@ -116,7 +116,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  return;
 	}
 #endif
-      ::new(static_cast<void*>(__p)) _Tp(std::forward<_Args>(__args)...);
+      ::new((void*)__p) _Tp(std::forward<_Args>(__args)...);
     }
 #else
   template<typename _T1, typename _T2>
@@ -132,7 +132,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _T1>
     inline void
     _Construct_novalue(_T1* __p)
-    { ::new(static_cast<void*>(__p)) _T1; }
+    { ::new((void*)__p) _T1; }
 
   template<typename _ForwardIterator>
     _GLIBCXX20_CONSTEXPR void

libstdc++-v3/testsuite/20_util/allocator/void.cc

@@ -87,8 +87,23 @@ static_assert( std::is_same<std::allocator<void>::const_pointer, const void*>(),
     "const_pointer is const void*" );
 #endif // C++20
 
+void
+test02()
+{
+  std::allocator<void> av;
+  int* p = std::allocator<int>().allocate(1);
+  const int* c = p;
+  std::allocator_traits<std::allocator<void>>::construct(av, c, 0);
+  volatile int* v = p;
+  std::allocator_traits<std::allocator<void>>::construct(av, v, 0);
+  const volatile int* cv = p;
+  std::allocator_traits<std::allocator<void>>::construct(av, cv, 0);
+  std::allocator<int>().deallocate(p, 1);
+}
+
 int
 main()
 {
   test01();
+  test02();
 }