Repair the <tuple> part of C++20 p1032 Misc constexpr bits.

2019-11-16  Edward Smith-Rowland  <3dw4rd@verizon.net>

	Repair the <tuple> part of C++20 p1032 Misc constexpr bits.
	* include/bits/uses_allocator.h (__uses_alloc0::_Sink::operaror=)
	(__use_alloc(const _Alloc&)) : Constexpr.

From-SVN: r278373

libstdc++-v3/ChangeLog

+2019-11-16  Edward Smith-Rowland  <3dw4rd@verizon.net>
+
+	Repair the <tuple> part of C++20 p1032 Misc constexpr bits.
+	* include/bits/uses_allocator.h (__uses_alloc0::_Sink::operaror=)
+	(__use_alloc(const _Alloc&)) : Constexpr.
+
 2019-11-17  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/std/string_view (basic_string_view(It, End)): Add range

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

@@ -72,7 +72,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   struct __uses_alloc0 : __uses_alloc_base
   {
-    struct _Sink { void operator=(const void*) { } } _M_a;
+    struct _Sink { void _GLIBCXX20_CONSTEXPR operator=(const void*) { } } _M_a;
   };
 
   template<typename _Alloc>
@@ -109,6 +109,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __uses_alloc<uses_allocator<_Tp, _Alloc>::value, _Tp, _Alloc, _Args...>;
 
   template<typename _Tp, typename _Alloc, typename... _Args>
+    _GLIBCXX20_CONSTEXPR
     inline __uses_alloc_t<_Tp, _Alloc, _Args...>
     __use_alloc(const _Alloc& __a)
     {