LWG3035 std::allocator's constructors should be constexpr

Moved 'constexpr' to after template-head.

Author: Dawn Perchik

source/utilities.tex

@@ -7470,9 +7470,9 @@
     using propagate_on_container_move_assignment = true_type;
     using is_always_equal = true_type;
 
-    allocator() noexcept;
-    allocator(const allocator&) noexcept;
-    template<class U> allocator(const allocator<U>&) noexcept;
+    constexpr allocator() noexcept;
+    constexpr allocator(const allocator&) noexcept;
+    template<class U> constexpr allocator(const allocator<U>&) noexcept;
     ~allocator();
 
     [[nodiscard]] T* allocate(size_t n);