[llvm] [APInt] Added APInt::clearBits() method (PR #137098)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

If you're willing, I'd recommend doing a separate cleanup PR to fix the setBits equivalent issues that @jayfoad identified first, then merge the changes (after they're accepted) into this clearBits patch - make sense?

https://github.com/llvm/llvm-project/pull/137098