Revert "Use owning pointers instead of raw pointers for Atom's to fix leaks."

This reverts commit r264022.

This breaks the Window's bots which don't like that i'm calling ~Atom when
the this pointer is a sublcass of Atom.

Reverting for now until I try find a better fix.  I tried using std::unique_ptr with
a custom deleter as a quick fix, but it didn't work well in the YAML parser.

llvm-svn: 264023