More work on ScalarExprEmitter::EmitCastExpr: for every cast kind, either

implement it explicitly or assert that it doesn't make sense for a scalar.
This caught a couple interesting issues: one, CK_BaseToDerivedMemberPointer
casts were getting silently miscompiled, and two, Sema was constructing some
strange implicit casts of type CK_UserDefinedConversion.

The change in SemaExprCXX makes sure the cast kinds are getting set correctly.

llvm-svn: 89987