[llvm] [Support][APInt] Fix sign extension, exponent and mantissa in APInt::roundToDouble (PR #192451)

[Andi Drebes via llvm-commits]

andidr wrote:

Thanks for the comments. I pushed a new version that delegates the conversion work to `APFloat::convertFromAPInt`. This is still implemented as a drop-in replacement for `roundToDouble` with the default rounding towards zero. Let me know if you would like to see the option for rounding exposed by the function.

https://github.com/llvm/llvm-project/pull/192451