[mlir][llvm] Support pointer entries in data layout translation

This adds support for pointer DLTI entries in LLVMIR export, e.g.
```
// translated to: p0:32:64:128
#dlti.dl_entry<!llvm.ptr, dense<[32,64,128]> : vector<3xi32>>
// translated to: p1:32:32:32:64
#dlti.dl_entry<!llvm.ptr<1>, dense<[32,32,32,64]> : vector<4xi32>>
```

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D133434