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

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Ariel-Burton wrote:

> That doesn't really answer either of my questions.

My apologies,  Let me try to be more explicit.

It's some time since I wrote format, and although I don't recall all the details, at the time I thought there was little that could be reused.  Scanning the routines again, the only thing that jumps out at me that might be easily sharable is the transformation from base 2 to base 10.  Not sure that's worth it.

With respect to considering to_chars-like functionality, that's out the scope of this change and represents a different policy.  This PR is intended to eliminate the conversion to host-format floating point while at the same time preserving the same output that hitherto would have been emitted.  In that sense we are trying to achieve a no-functional-change.  So, yes, we could consider doing something like to_chars(), but not in this change.

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