[GlobalOpt] Emit fragments using field offsets from struct layout

Summary:
When creating the debug fragments for a SRA'd struct, use the fields'
offsets, taken from the struct layout, as the offsets for the resulting
fragments. This fixes an issue where GlobalOpt would emit fragments with
incorrect offsets for padded fields.

This should solve PR36016.

Patch by David Stenberg.

Reviewers: aprantl

Reviewed By: aprantl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D42489

llvm-svn: 323411