[llvm] [msan] Add experimental '-msan-or-shadow-for-strict-instructions' flag to pessimize output (PR #128036)

[Thurston Dang via llvm-commits]

thurstond wrote:

> > This patch also does a drive-by fix of the ClCheckConstantShadow description, to note that constant zero shadows are never checked.
> 
> Why in this patch?

This patch deals with constant zero shadows.

https://github.com/llvm/llvm-project/pull/128036