Revert "[ELF] Create input and output section names"

This reverts commit r222311.

Reverting because of failure in the darwin bot.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222329 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "[ELF] Rename MergedSection to OutputSection."

This reverts commit r222310.

Not sure which commit is the cause of the failure on the darwin bot. Will need
to revert my changes and commit one change at a time.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222330 91177308-0d34-0410-b5e6-96231b3b80d8