gccrs: ast: Dump bare function type

+ Return FunctionQualifiers as ref to work in ast dump

gcc/rust/ChangeLog:

	* ast/rust-ast-dump.cc (Dump::visit): Add missing visitor.
	* ast/rust-ast-dump.h: Add missing getter declaration.
	* ast/rust-ast-full-test.cc (BareFunctionType::as_string): Fix bare function
	string representation.
	* ast/rust-type.h (class BareFunctionType): Declare said getter.

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