Fix a wrong comment.

Previously FileArchive ctor comment said that only its subclasses
can be instantiated, but the ctor is actually public and is
instantiated by ArchiveReader.

Remove the wrong comment and reorder the member functions so that
public members appear before private ones.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@210175 91177308-0d34-0410-b5e6-96231b3b80d8