[llvm] [ADT] Fix a bug in PackedVector::setValue for signed types (PR #159239)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

I'd think that in order to support signed types, we have to take care of sign extension and that merely manipulating the MSB is not enough? I haven't looked at the logic carefully, but this sounds fishy...

https://github.com/llvm/llvm-project/pull/159239