[llvm] [NFC] [MSAN] disambiguate insertShadowCheck (PR #146616)

[Thurston Dang via llvm-commits]

thurstond wrote:

IMO the new names (`insertShadowCheck`, `insertCheckShadow`) are so similar that it becomes confusing which one should be used
e.g. does Check shadow(X,...) mean check the shadow of X, or check the shadow X?

Maybe something like insertCheckShadowOf(X)?

https://github.com/llvm/llvm-project/pull/146616