[clang-tools-extra] [Support] Add KnownBits::computeForSubBorrow (PR #67788)

[Christian Kissig via cfe-commits]

christiankissig wrote:

> Might be nice to add a test where this actually shows up in CodeGen.

+1 Will look to add it with this change.

https://github.com/llvm/llvm-project/pull/67788