c++: Accept the use of built-in trait identifiers

c++: Accept the use of built-in trait identifiers

This patch accepts the use of built-in trait identifiers when they are
actually not used as traits.  Specifically, we check if the subsequent
token is '(' for ordinary built-in traits or is '<' only for the special
__type_pack_element built-in trait.  If those identifiers are used
differently, the parser treats them as normal identifiers.  This allows
us to accept code like: struct __is_pointer {};.

gcc/cp/ChangeLog:

	* parser.cc (cp_lexer_lookup_trait): Rename to ...
	(cp_lexer_peek_trait): ... this.  Handle a subsequent token for
	the corresponding built-in trait.
	(cp_lexer_lookup_trait_expr): Rename to ...
	(cp_lexer_peek_trait_expr): ... this.
	(cp_lexer_lookup_trait_type): Rename to ...
	(cp_lexer_peek_trait_type): ... this.
	(cp_lexer_next_token_is_decl_specifier_keyword): Call
	cp_lexer_peek_trait_type.
	(cp_parser_simple_type_specifier): Likewise.
	(cp_parser_primary_expression): Call cp_lexer_peek_trait_expr.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>