Promote MemRefDescriptor to a pointer to struct when passing function boundaries in LLVMLowering.

The strided MemRef RFC discusses a normalized descriptor and interaction with library calls (https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/MaL8m2nXuio).
Lowering of nested LLVM structs as value types does not play nicely with externally compiled C/C++ functions due to ABI issues.
Solving the ABI problem generally is a very complex problem and most likely involves taking
a dependence on clang that we do not want atm.

A simple workaround is to pass pointers to memref descriptors at function boundaries, which this CL implement.

PiperOrigin-RevId: 271591708