gccrs: Make use of the Contextual visitor in validation

gccrs: Make use of the Contextual visitor in validation

Use the new contextual ast visitor to reduce the amount of code in the
ast validation visitor.

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::visit): Adapt
	the call to the new visit functions.
	(ASTValidation::check): Launch the parent class visitor root function.
	* checks/errors/rust-ast-validation.h (class ASTValidation): Inherit
	from the contextual visitor.

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