[mlir][llvm] Add structured loop metadata

This commit introduces a structured representation of loop metadata to
the LLVM dialect. This attribute explicitly models all known `!llvm.loop`
metadata fields and groups them by introducing nested attributes for each
namespace.

The new attribute replaces the LoopOptionAttr that could only model a
limited subset of loop metadata.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D143064