[mlir][llvm] Import access group metadata.

The revision adds support to import access group metadata from LLVM IR.
It closely follows the design of the TBAA metadata import with an
up-front conversion of the metadata nodes to operations stored in the
body of a module-level metadata operation. The revision chooses to use
only one module-level metadata operation for all kinds of metadata.
This  design ensures there is only one metadata operation that pollutes
the user namespace.

The import of loop metadata, which will use the access groups,
is left to a follow up revision.

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D142605