[llvm] [msan] Fix false negatives from partially undefined constant fixed-length vectors (PR #143837)

[Thurston Dang via llvm-commits]

thurstond wrote:

> LGTM but please wait until we resolve the current regression

Can I land it earlier if I change the fix to be off by default (controlled by the new flag)? (And then eventually flip the flag on by default in a separate CL.)

The advantages are:
- once the fix lands (disabled by default) in toolchains, it's easier to let owners of buggy code re-run MSan with the flag enabled, rather than telling them they need to custom patch the compiler
- the fix will be lightly tested in CI by the new test case

https://github.com/llvm/llvm-project/pull/143837