[ELF] Fix layout of output sections.

The AtomSections were improperly merging sections from various input files. This
patch fixes the problem, with an updated test that was provided by Simon.

Thanks to Simon Atanasyan for catching this issue.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222982 91177308-0d34-0410-b5e6-96231b3b80d8