Disable auto_is_implicit_function_template_parm_p while parsing attributes

Disable auto_is_implicit_function_template_parm_p while parsing attributes

We don't want functions to become implicit templates just because of
random uses of auto in unexpected places.  Disabling the special
handling of auto while parsing attributes, for example, makes for
more sensible errors.

for  gcc/cp/ChangeLog

	PR c++/84610
	PR c++/84642
	PR c++/84942
	* cp-tree.h (temp_override): New template class, generalizing
	a cleanup that was only used...
	* parser.c (cp_parser_parameter_declaration_clause):
	... here for auto_is_implicit_function_template_parm_p.
	(cp_parser_gnu_attributes_opt): Use it here as well.
	(cp_parser_std_attribute): Likewise.

From-SVN: r258790