[clang] [clang][Sema] Move computing enum bits into a separate function (PR #126096)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> The code needs to be called from LLDB as well (#115005), 

Please add this context to the patch summary so that it explains why the changes are needed, not just what the changes are doing.

https://github.com/llvm/llvm-project/pull/126096