Normalize lowering of MemRef types

The RFC for unifying Linalg and Affine compilation passes into an end-to-end flow with a predictable ABI and linkage to external function calls raised the question of why we have variable sized descriptors for memrefs depending on whether they have static or dynamic dimensions  (https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/MaL8m2nXuio).

This CL standardizes the ABI on the rank of the memrefs.
The LLVM struct for a memref becomes equivalent to:
```
template <typename Elem, size_t Rank>
struct {
  Elem *ptr;
  int64_t sizes[Rank];
};
```

PiperOrigin-RevId: 270947276