Add AST validation check for const in trait

Add AST validation check for const in trait

Add a new check in AST validation pass that checks that no function
declaration in traits are declared const.

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::visit): Add
	const check.
	* checks/errors/rust-ast-validation.h: Add visit function prototype.

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