[llvm-objcopy] - Fix --compress-debug-sections when there are relocations.

When --compress-debug-sections is given,
llvm-objcopy removes the uncompressed sections and adds compressed to the section list.
This makes all the pointers to old sections to be outdated.

Currently, code already has logic for replacing the target sections of the relocation
sections. But we also have to update the relocations by themselves.

This fixes https://bugs.llvm.org/show_bug.cgi?id=40885.

Differential revision: https://reviews.llvm.org/D58960

llvm-svn: 355821