[libc++][NFC] Increase readability of typeinfo comparison of ARM64

We wasted a good deal of time trying to figure out whether our implementation
was correct. In the end, it was, but it wasn't so easy to determine. This
patch dumbs down the implementation and improves the documentation to make
it easier to validate.

See https://lists.llvm.org/pipermail/libcxx-dev/2020-December/001060.html.

Differential Revision: https://reviews.llvm.org/D97802