[PECOFF] Fix alignment bug.

There was a bug that if a section has an alignment requirement and there are
multiple symbols at offset 0 in the section, only the last atom at offset 0
would be aligned properly. That bug would move only the last symbol to an
alignment boundary, leaving other symbols unaligned, although they should be at
the same location. That caused a mysterious SEGV error of the resultant
executable.

With this patch, we manage all symbols at the same location properly, rather
than keeping the last one.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190724 91177308-0d34-0410-b5e6-96231b3b80d8