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

[Vitaly Buka via llvm-commits]

vitalybuka 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?

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