Reuse a technique (pioneered for BasicBlocks) of superposing ilist with

its sentinel. This is quite a win when a function really has a basic block.                  
When the function is just a declaration (and stays so) the old way did not
allocate a sentinel. So this change is most beneficial when the ratio of
function definition to declaration is high. I.e. linkers etc. Incidentally  
these are the most resource demanding applications, so I expect that the
reduced malloc traffic, locality and space savings outweigh the cost of
addition of two pointers to Function.

llvm-svn: 65776