[mlir][LLVM] Convert access group metadata to using attributes instead of ops

Using MLIR attributes instead of metadata has many advantages:
* No indirection: Attributes can simply refer to each other seemlessly without having to use the indirection of `SymbolRefAttr`. This also gives us correctness by construction in a lot of places as well
* Multithreading safe: The Attribute infrastructure gives us thread-safety for free. Creating operations and inserting them into a block is not thread-safe. This is a major use case for e.g. the inliner in MLIR which runs in parallel
* Easier to create: There is no need for a builder or a metadata region

This patch therefore does exactly that. It leverages the new distinct attributes to create distinct access groups in a deterministic and threadsafe manner.

Differential Revision: https://reviews.llvm.org/D155285