[llvm] [APFloat] Add APFloat::format (PR #99088)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

Can any of the code be shared with toString?

Do we really want to mimic the old llvm::write_double API, as opposed to doing something closer to std::to_chars?

https://github.com/llvm/llvm-project/pull/99088