[llvm-c] Harmonize usage of unwrap (NFC)

Some places were using unwrap<T>(x) = cast<T>(unwrap(x)), even though
the unwrapped value already had type T. Removing the template argument
makes it clear that no cast is intended.

Other places were using cast<T>(unwrap(x)), we replace that with the
shorthand unwrap<T>(x) for consistency.