based on constant. prop. and limited symbolics.

- Renamed class: RValue -> RVal, LValue -> LVal, etc.
- Minor method renamings and interface cleanups.
- Tightened the RVal "type system" so that UninitializedVal and UnknownVal
  cannot be cast to LVal or NonLVal.  This forces these corner cases values
  to be explicitly handled early before being dispatched to plug-in transfer
  function logic.
- Major cleanup in the transfer function logic for binary and unary operators.
  Still fixing some regressions, but we now explicitly handle Uninitialized
  and Unknown values in a more rigorous way.

llvm-svn: 47441