Revert D63432 "[ELF] Allow placing SHF_MERGE sections with different...

Revert D63432 "[ELF] Allow placing SHF_MERGE sections with different alignments into the same MergeSyntheticSection"

This reverts r365015.

David Zarzycki reported this change broke stage2 and stage3 tests.  The
root cause is still not very clear, but I guess some SHF_MERGE sections
with the same name have different alignments. They were not merged
before but were merged after r365015.

Something that assumes address uniqueness of such mergeable data caused
the bug.

llvm-svn: 365048