Parse const with no value expression

Parse const with no value expression

Const with no value expression may exist either in trait or in disabled
blocks. This means we should be able to parse those correctly.

gcc/rust/ChangeLog:

	* ast/rust-item.h: Add a new constructor for const with no value
	expression.
	* parse/rust-parse-impl.h (Parser::parse_const_item): Allow const with
	no expression value.

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