std.cfg: Added support for std::[oi]fstream::is_open() (#5997)

Author: orbitcowboy

cfg/std.cfg

@@ -6536,6 +6536,13 @@ The obsolete function 'gets' is called. With 'gets' you'll get a buffer overrun
     <returnValue type="bool"/>
     <noreturn>false</noreturn>
   </function>
+  <!-- bool is_open() const; -->
+  <function name="std::basic_ifstream::is_open,std::ifstream::is_open,std::basic_ofstream::is_open,std::ofstream::is_open">
+    <use-retval/>
+    <const/>
+    <returnValue type="bool"/>
+    <noreturn>false</noreturn>
+  </function>
   <!--std::ofstream& std::ofstream::write (const char* s, streamsize n);-->
   <function name="std::ofstream::write">
     <returnValue type="std::ofstream &amp;"/>