[OpenMP] Use Undef instead of null as pointer for inactive lanes

Our conditional writes in the runtime look like this:
```
  if (active)
    *ptr = value;
```
In the RAII we need to assign `ptr` which comes from a lookup call.
If a thread that is not the main thread calls lookup with the intention
to write the pointer, we'll create a new thread state. As such, we need
to avoid calling lookup for inactive threads. We used to use `nullptr`
as their `ptr` value but that can cause pessimistic reasoning. We now
use `undef` instead.

Differential Revision: https://reviews.llvm.org/D130114

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