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

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Sirraide wrote:

> Don't really know where exactly to place it inside `ASTContext.h`, though.

Honestly, just put it wherever I’d say. Unlike `Sema.h`, `ASTContext.h` is much less well-organised last time I checked.

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