[mlir] Handle different pointer sizes in unranked memref descriptors

The code for unranked memref descriptors assumed that
sizeof(!llvm.ptr) == lizeof(!llvm.ptr<N>) for all address spaces N.
This is not always true (ex. the AMDGPU compiler backend has
sizeof(!llvm.ptr) = 64 bits but sizeof(!llvm.ptr<5>) = 32 bits, where
address space 5 is used for stack allocations). While this is merely
an overallocation in the case where a non-0 address space has pointers
smaller than the default, the existing code could cause OOB memory
accesses when sizeof(!llvm.ptr<N>) > sizeof(!llvm.ptr).

So, add an address spaces parameter to computeSizes in order to
partially resolve this class of bugs. Note that the LLVM data layout
in the conversion passes is currently set to "" and not constructed
from the MLIR data layout or some other source, but this could change
in the future.

Depends on D142159

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D141293