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

[Ilia Kuklin via cfe-commits]

kuilpd wrote:

As of now LLDB doesn't calculate enum's best promotion type when reading it from DWARF info. This change would allow to reuse the code from Sema without copying it to LLDB, which I'm working on in #115005 
This function however has too many arguments right now, and also needs to return 3 results, so I'd be grateful for some advice how to make it look better.

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