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

[Ilia Kuklin via cfe-commits]

kuilpd wrote:

Tried moving the code to `EnumDecl`, but it didn't work as well as I thought, so moved it to `ASTContext` after all. Don't really know where exactly to place it inside `ASTContext.h`, though.

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