Revert "[llvm-reduce] Remove debug metadata elements"

This reverts commit 69549de8.

The change in D135237 can lead to verification failures like `scope must have two or three operands`.
The ongoing work in D132077 does something similar without these failures, so lets wait for that to land and revert this patch.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D135395