[OpenMP] Avoid creating null pointer lvalue (NFC)

The reduction initialization code creates a "naturally aligned null
pointer to void lvalue", which I found somewhat odd, even though it
works out in the end because it is not actually used. It doesn't
look like this code actually needs an LValue for anything though,
and we can use an invalid Address to represent this case instead.

Differential Revision: https://reviews.llvm.org/D116214