[llvm] [APFloat] Fix `IEEEFloat::addOrSubtractSignificand` and `IEEEFloat::normalize` (PR #98721)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

`ÌEEEFloat` is currently `final`. Could make it not final with a comment: "not final for testing".
Make `addOrSubtractSignificand` protected. Then you can inherit from `IEEEFloat` for testing.

https://github.com/llvm/llvm-project/pull/98721