Remove default argument for ImpCastExprToType. Add appropriate argument

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@84623 91177308-0d34-0410-b5e6-96231b3b80d8