gdb: add string_file::release method

A common pattern for string_file is to want to move out the internal
string buffer, because it is the result of the computation that we want
to return.  It is the reason why string_file::string returns a non-const
reference, as explained in the comment.  I think it would make sense to
have a dedicated method for that instead and make string_file::string
return a const reference.

This allows removing the explicit std::move in the typical case.  Note
that compile_program::compute was missing a move, meaning that the
resulting string was copied.  With the new version, it's not possible to
forget to move.

Change-Id: Ieaefa35b73daa7930b2f3a26988b6e3b4121bb79

gdb/cli/cli-setshow.c

@@ -659,7 +659,7 @@ get_setshow_command_value_string (const setting &var)
       gdb_assert_not_reached ("bad var_type");
     }
 
-  return std::move (stb.string ());
+  return stb.release ();
 }