[AppleAccelTable][NFC] Improve code readability

This commit does a few minor NFC cleanups:

* A variable was called "Atom", probably trying to claim it was an AtomType.
This was incorrect, it is actually a FormValue.

* LLVM provides a `zip_equal` to express the intent of asserting ranges with the
same size. We change the lookup method to use that.

* The use of tuples made the code slightly difficult to follow, as such we
unpack the tuple with structure binding to improve readability.

Depends on D152157

Differential Revision: https://reviews.llvm.org/D152158