Separate atom_collection type into two different types. NFC.

atom_collection is basically a wrapper for std::vector. The class
provides begin and end member functions, so that it "hides" the
other member functions provided by std::vector. However, you can
still directly access _atoms member since the member is not
protected.

We cannot simply make the member private because we need that member
when we are constructing atom vectors.

This patch splits atom_collection into two types: std::vector<Atom *>
and AtomRange. When we are constructing atom vectors, we use the
former class. We return instances of the latter class from File
objects so that callers cannot add or remove atoms from the lists.

std::vector<Atom *> is automatically converted to AtomRange.

llvm-svn: 234450