[ELF] Rename MergedSection to OutputSection.

No change in functionality.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222972 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Create input and output section names

No change in functionality.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222973 91177308-0d34-0410-b5e6-96231b3b80d8