[mlir][llvm] Purge struct_attr

This commit removes the `llvm.struct_attr` which was used to bundle
result attributes that were previously attached to multiple results.
This extension isn't part of LLVM as result attribute semantics cannot
be supported on a struct field granularity.
Furthermore, many usages promoted result attributes to argument
attributes but this does not necessary preserve the semantics.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D143473