clang: Avoid a hot FFI call for getting the cursor kind.
clang: Avoid a hot FFI call for getting the cursor kind.

This is a sorta-hot call already.

I've noticed #544 wants to add more assertions about this, and was going to
suggest moving them to `debug_assert!`.

But there's an easier way :)

Signed-off-by: Emilio Cobos Álvarez <86fa523305b8ad6a8a853c04cfef24a0445908d3@crisal.io>

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