gccrs: Emit an error on malformed path

gccrs: Emit an error on malformed path

Path must be made of a single literal item, otherwise an error should be
thrown.

gcc/rust/ChangeLog:

	* util/rust-attributes.cc (AttributeChecker::check_attribute): Recurse
	within attr input for additional attribute checking.
	(AttributeChecker::visit): Remove empty definition in favor of default
	ast visitor definition.
	* util/rust-attributes.h: Remove now unused prototypes.

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