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

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https://github.com/Sirraide commented:

This is just moving stuff around so that seems fine. Though, I wonder, could we just pass e.g. an `ArrayRef<Decl*>` to `computeEnumBits()`? The LLDB pr you linked seems to have an `SmallVector<EnumConstantDecl*>, so that should be possible (I think it does require a `reinterpret_cast` from a `const Decl**` to a `const EnumConstantDecl**`, but we already do that in other places in Clang iirc).

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