[llvm] [APFloat] Add `APFloat::getConstant` for C++20 <numbers> constants (PR #227916)

[Eric Ross via llvm-commits]

ZERICO2005 wrote:

> I used this script to generate the correctly rounded constants https://pastebin.com/WTePdEAW

For future reference, should I leave the link to the script in the PR description, or as a comment under the PR?

https://github.com/llvm/llvm-project/pull/227916