[mlir][llvm] Fuse MD_access_group & MD_loop import

This commit moves the importing logic of access group metadata into the
loop annotation importer. These two metadata imports can be grouped
because access groups are only used in combination with
`llvm.loop.parallel_accesses`.

As a nice side effect, this commit decouples the LoopAnnotationImporter
from the ModuleImport class.

Differential Revision: https://reviews.llvm.org/D143577