ir: When something has a definition, return unresolved type references until we parse it.
ir: When something has a definition, return unresolved type references until we parse it.

This ensures that we see all the relevant types that are defined when parsing
the definition, avoiding problems like #888.

Fixes #888

emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen