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

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