gccrs: ast: add visit overload for references

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

gcc/rust/ChangeLog:

	* ast/rust-ast-dump.cc (Dump::visit): Add new reference visitor wrapper.
	* ast/rust-ast-dump.h: Declare it.
	* ast/rust-item.h: Add mutable visibility getters.

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