Add ICE regression test with unboxed closures

This code used to produce the following ICE:

    error: internal compiler error: get_unique_type_id_of_type() -
    unexpected type: closure,
    ty_unboxed_closure(syntax::ast::DefId{krate: 0u32, node: 66u32},
    ReScope(63u32))

This is a regression test for issue #17021.