[mlir][llvm] Fuse access_group & loop export (NFC)

This commit moves the access group translation into the
LoopAnnotationTranslation class as these two metadata kinds only appear
together.

Drops the access group cleanup from `ModuleTranslation::forgetMapping`
as this is only used on function regions. Access groups only appear in the
region of a global metadata operation and will thus not be cleaned here.

Analogous to https://reviews.llvm.org/D143577

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D144253