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/ada-lang.c

@@ -6915,7 +6915,7 @@ type_as_string (struct type *type)
 
   type_print (type, "", &tmp_stream, -1);
 
-  return std::move (tmp_stream.string ());
+  return tmp_stream.release ();
 }
 
 /* Given a type TYPE, look up the type of the component of type named NAME.