Allow const and async specifiers in functions

Allow const and async specifiers in functions

We need to account for const specifiers in async parsing as const
can be used in the syntax before the async keyword.

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_vis_item): Allow parsing async
	items in const.
	(Parser::parse_async_item): Account for const offset during async
	lookahead.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>