A couple of big refactorings: 1) Move most attributes of Atom down to...

A couple of big refactorings: 1) Move most attributes of Atom down to DefinedAtom, so only atoms representing definitions need to implement them. 2) Remove definitionTentative, definitionWeak, mergeDuplicates, and autoHide.  Replace with merge and interposable attributes. 3) Make all methods on Atom be virtual so that future object file readers can lazily generated attributes

llvm-svn: 147903