[Attributor][FIX] Ensure unknown accesses are honored

If we had an unknown access but already some prior knowledge (known), we
could have ended up ignoring the unknown access all together. The
problem is that we track unknown not as all locations but separately.
This patch bridges the gap and expands the unknown bits to "all bits"
when we add an access.

Fixes: https://github.com/llvm/llvm-project/issues/63291