Annotations are evil. This makes Value not derive from Annotable, which makes

all dynamically allocated LLVM values 4 bytes smaller, eliminate some vtables, and
make Value's destructor faster.

This makes Function derive from Annotation now because it is the only core LLVM
class that still has an annotation stuck onto it: MachineFunction.
MachineFunction is obviously horrible and gross (like most other annotations), but
will be the subject of refactorings later in the future.  Besides many fewer
Function objects are dynamically allocated that instructions blocks, constants,
types, etc... :)

llvm-svn: 11878