[clang] [clang:static-analyzer] fix: allow construction of atomic pointers from nullptr. (PR #190131)

[Balázs Benics via cfe-commits]

https://github.com/steakhal approved this pull request.

Looks good to me.
One question I had while reading this for the second time was:

If we would canonicalize before the assert in `makeNullWithType`, then we wouldn't need to again canonicalize in `isLocType`.

My concern is that `isLocType` would now start accepting Atomic types, while there could be potential callsites not doing this canonicalization.
While I think the lack of static guarantees is not ideal, we are definitely better off with having this patch than not. Ideally, we should enforce canonicalization preventing this bug class.

@NagyDonat WDYT?

https://github.com/llvm/llvm-project/pull/190131