std.cfg: Add mem_fn() (#1976)

std.cfg: Add mem_fn()

Reference:
https://en.cppreference.com/w/cpp/utility/functional/mem_fn

cfg/std.cfg

@@ -7502,6 +7502,16 @@ initializer list (7) string& replace (const_iterator i1, const_iterator i2, init
     <arg nr="1"/>
     <arg nr="2"/>
   </function>
+  <!-- template< class M, class T > /*unspecified*/ mem_fn(M T::* pm); // since C++11 until C++17 -->
+  <!-- template< class M, class T > /*unspecified*/ mem_fn(M T::* pm) noexcept; // since C++17 -->
+  <function name="std::mem_fn">
+    <noreturn>false</noreturn>
+    <use-retval/>
+    <arg nr="1" direction="in">
+      <not-null/>
+      <not-uninit/>
+    </arg>
+  </function>
   <memory>
     <alloc init="false" buffer-size="malloc">malloc</alloc>
     <alloc init="true" buffer-size="calloc">calloc</alloc>