Revert "Reland D64327 [MC][ELF] Allow STT_SECTION referencing SHF_MERGE on REL targets"
This reverts commit b497665d98ad5026b1d3d67d5793a28fefe27bea.
Spent some time trying to reproduce this locally, reverting in a
desparate attempt to fix the sanitizer buildbot:
I don't know exactly why or how this patch breaks the bots, but it seems
pretty concrete that it's the culprit.