Implement LWG 2946, 3075 and 3076. Reviewed as https://reviews.llvm.org/D48616

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@336132 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.nonmembers/string.special/swap_noexcept.pass.cpp

@@ -35,8 +35,8 @@ struct some_alloc
 
     some_alloc() {}
     some_alloc(const some_alloc&);
+    T *allocate(size_t);
     void deallocate(void*, unsigned) {}
-
     typedef std::true_type propagate_on_container_swap;
 };
 
@@ -47,6 +47,7 @@ struct some_alloc2
 
     some_alloc2() {}
     some_alloc2(const some_alloc2&);
+    T *allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::false_type propagate_on_container_swap;