Add binding context class

Add binding context class

We need to differentiate bindings types, so the same binding cannot be
reused multiple time in a product binding.

gcc/rust/ChangeLog:

	* resolve/rust-name-resolution-context.h (struct Binding): Add Binding
	struct to differentiate Or and Product bindings in patterns.
	(enum class): Add Binding kind.
	(class BindingContext): Add binding context with Binding stack.

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