[ADT] Fix a bug in DoubleAPFloat::frexp (#161625)
Without this patch, we call APFloat::makeQuiet() in frexp like so:
Quiet.getFirst().makeQuiet();
The problem is that makeQuiet returns a new value instead of modifying
"*this" in place, so we end up discarding the newly returned value.
This patch fixes the problem by assigning the result back to
Quiet.getFirst().
We should put [[nodiscard]] on APFloat::makeQuiet, but I'll do that in
another patch.
