Remove default argument for ImpCastExprToType. Add appropriate argument

to all callers.  Switch a few other users of CK_Unknown to proper cast 
kinds.

Note that there are still some situations where we end up with 
CK_Unknown; they're pretty easy to find with grep. There 
are still a few missing conversion kinds, specifically 
pointer/int/float->bool and the various combinations of real/complex 
float/int->real/complex float/int.

llvm-svn: 84623