Add parens around constraint in <memory>.

C++20 concepts require parens around atomic constraints that are not
primary-expressions.

	* include/std/memory (uses_allocator_construction_args): Add parens
	around constraint.

From-SVN: r273515

libstdc++-v3/ChangeLog

+2019-07-16  Jason Merrill  <jason@redhat.com>
+
+	* include/std/memory (uses_allocator_construction_args): Add parens
+	around constraint.
+
 2019-07-12  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/29_atomics/atomic_float/1.cc: Fix comment.

libstdc++-v3/include/std/memory

@@ -261,7 +261,7 @@ get_pointer_safety() noexcept { return pointer_safety::relaxed; }
     uses_allocator_construction_args(const _Alloc& __a,
 				     _Args&&... __args) noexcept
 #if __cpp_concepts
-    requires ! _Std_pair<_Tp>
+    requires (! _Std_pair<_Tp>)
 #endif
     {
       return std::__uses_alloc_args<_Tp>(__a, std::forward<_Args>(__args)...);