[llvm] [APFloat] Signal overflow when rounding delivers the largest finite value (PR #220020)

[Jack Royer via llvm-commits]

Garfield1002 wrote:

The formatter reformats the entire enclosing `SpecialCaseTests` table since I changed some row.
This produces a significantly larger unrelated diff.

I'm leaving the simple diff for now, let me know if you want me to apply the formatting changes.

https://github.com/llvm/llvm-project/pull/220020