gccrs: ast: Dump where clause and recursively needed nodes

This is currently needed for lifetimes to use the existing infrastructure.

gcc/rust/ChangeLog:

	* ast/rust-ast-dump.cc (Dump::visit): Add missing visitors.
	* ast/rust-ast-dump.h: Likewise.
	* ast/rust-ast.h: Add `get_lifetime_bounds` method.
	* ast/rust-item.h: Add missing getter for lifetimes.
	* ast/rust-type.h: Likewise.

Signed-off-by: Jakub Dupak <dev@jakubdupak.com>