c++: Parser tweaks

c++: Parser tweaks

We need to adjust the wording for 'export'.  Between c++11 and c++20
it is deprecated.  Outside those ranges it is unsupported (at the
moment).  While here, there's also an unneeded setting of a bool --
it's inside an if block that just checked it was true.

	gcc/cp/
	* parser.c (cp_parser_template_declaration): Adjust 'export' warning.
	(cp_parser_explicit_specialization): Remove unneeded bool setting.