gccrs: Emit an error on associated const without values

gccrs: Emit an error on associated const without values

Associated const with no value that are not in trait impl are prohibited.

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::check): Launch
	check over the whole given crate.
	(ASTValidation::visit): Implement visitor for some members of the ast.
	* checks/errors/rust-ast-validation.h: Update some prototype according
	to implemented visitor functions. Also add a context tracker.

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