Add visibility to trait item

Add visibility to trait item

The compiler shall parse visibility modifiers on trait items and reject
those at a later stage (ast validation).

gcc/rust/ChangeLog:

	* ast/rust-item.h (struct Visibility): Move Visibility from here...
	* ast/rust-ast.h (struct Visibility): ...to here.
	* parse/rust-parse-impl.h (Parser::parse_trait_item): Parse visibility
	before giving it back to the item parsing function.
	(Parser::parse_trait_type): Add visibility modifier.
	* parse/rust-parse.h (RUST_PARSE_H): Change function prototype.

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