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

[Thorsten Schütt via llvm-commits]

================
@@ -306,7 +306,8 @@ struct APFloatBase {
 
 namespace detail {
 
-class IEEEFloat final : public APFloatBase {
+// Not final for testing purposes only.
----------------
tschuett wrote:

`addOrSubtractSignificand` went from private to protected. It is the only non-fishy way to get access for testing.

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