[XCOFF] Clean-up enum use in BinaryFormat/XCOFF.h; NFC

Summary:
This patch splits mostly unrelated size constants into separate
constexpr variables, sets explicit underlying types for the enumerations
to match the fields they are used for, and improves various comments.

This patch also replaces `<cname>` headers with `<name.h>` headers to
match the usage of the declared names as global namespace members in the
file.

Reviewers: jasonliu, DiggerLin, daltenty, sfertile

Reviewed By: jasonliu, sfertile

Differential Revision: https://reviews.llvm.org/D79136