expr2c: Add justifying comment for use of source_locationt::get_function()

Even though we use it as an index into a data structure, ambiguity is not a
problem here.

Author: tautschnig

src/ansi-c/expr2c.cpp

@@ -160,6 +160,8 @@ void expr2ct::get_shorthands(const exprt &expr)
       irep_idt func;
 
       const symbolt *symbol;
+      // we use the source-level function name as a means to detect collisions,
+      // which is ok, because this is about generating user-visible output
       if(!ns.lookup(symbol_id, symbol))
         func=symbol->location.get_function();