[clang] [clang][Sema] Move computing enum width and type to a separate function (PR #120965)

[Michael Buch via cfe-commits]

https://github.com/Michael137 commented:

Looks fine to me, thanks! I'll defer approval to one of the maintainers

(lets mark the commit as `[NFC]` so it's clear that this doesn't change behaviour)

https://github.com/llvm/llvm-project/pull/120965