* linespec.c (struct linespec): Constify expression,

	* linespec.c (struct linespec): Constify expression,
	source_filename, function_name, and label_name.
	(symbol_not_found_error): Make all parameters const.
	(linespec_parser_delete): No need to check for NULL
	when using xfree. Cast const char * to char * for xfree.