OpenMP: Robustify C front end handling of attribute-syntax pragmas

Presently, the code to handle OpenMP attribute-syntax pragmas in the C
front end assumes nothing else is messing with redirecting
parser->tokens, and makes no provision for restoring it from anything
other than parser->tokens_buf when the buffer allocated for the pragma
is exhausted.  Adding support for metadirectives will change that,
since it also needs to buffer tokens for the metadirective
alternatives, and an attribute-syntax directive can appear inside a
metadirective.

This patch adds a more general save/restore mechanism attached to the
parser via a pointer to a new struct omp_attribute_pragma_state.

gcc/c/ChangeLog

	* c-parser.cc (struct c_parser): Change in_omp_attribute_pragma
	field to be of type struct omp_attribute_pragma_state.
	(struct omp_attribute_pragma_state): New.
	(c_parser_skip_until_found): Use the new way to restore state
	on EOF.
	(c_parser_skip_to_pragma_eol): Likewise.
	(c_parser_handle_statement_omp_attributes): Create an
	omp_attribute_pragma_state to hold the restore state.  Do not
	store state in tok.flags.
	(omp_maybe_parse_omp_decl): Likewise.