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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@89987 91177308-0d34-0410-b5e6-96231b3b80d8