[DebugInfo] Update loop metadata in stripNonLineTableDebugInfo

Summary:
Have stripNonLineTableDebugInfo() attach updated !llvm.loop metadata to
an instruction (instead of updating and then discarding the metadata).

This fixes "!dbg attachment points at wrong subprogram for function"
errors seen while archiving an iOS app.

It would be nice -- as a follow-up -- to catch this issue earlier,
perhaps by modifying the verifier to constrain where DILocations are
allowed. Any alternative suggestions appreciated.

rdar://61982466

Reviewers: aprantl, dsanders

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79200