Currently to get the flat address space, we have to query it via TargetTransformInfo because getFlatAddressSpace is a member function of TargetTransformInfo. The problem with it is, flat address space is really just a target related feature, and it is not associated with any function. Having it in TargetTransformInfo means we have to have a Function & to query that information, which in some cases is not available.
I have proposed a PR to add it into DataLayout, then we can remove it from TargetTransformInfo.