Add support for alignment attribute in std.alloc.

This CL adds an extra pointer to the memref descriptor to allow specifying alignment.

In a previous implementation, we used 2 types: `linalg.buffer` and `view` where the buffer type was the unit of allocation/deallocation/alignment and `view` was the unit of indexing.

After multiple discussions it was decided to use a single type, which conflates both, so the memref descriptor now needs to carry both pointers.

This is consistent with the [RFC-Proposed Changes to MemRef and Tensor MLIR Types](https://groups.google.com/a/tensorflow.org/forum/#!searchin/mlir/std.view%7Csort:date/mlir/-wKHANzDNTg/4K6nUAp8AAAJ).

PiperOrigin-RevId: 279959463