[llvm] [msan][NFCI] Generalize handleIntrinsicByApplyingToShadow to allow alternative intrinsic for shadows (PR #124831)

[Thurston Dang via llvm-commits]

thurstond wrote:

> #124662 is not urgent to fix

Ack. It'll only take ~5 minutes though so I'll clean it up.

https://github.com/llvm/llvm-project/pull/124831