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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

What exactly is different between toString and format?  Both routines support both fixed and scientific-style output, and they both use a constant "precision".  There are minor differences in some specific usages, but I'd rather slightly extend the toString API, instead of maintaining two implementations of string conversion.

> With respect to considering to_chars-like functionality

You mean, the round-trip precision stuff?  Sure, we don't need to do that here.

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