For ObjCXX, we can create a CastExpr with Kind being CK_UserDefinedConversion
and SubExpr being BlockExpr. Specifically one can return BlockExpr from
BuildCXXMemberCallExpr and the result can be used to build a CastExpr.
Fix the assumption in CastExpr::getSubExprAsWritten that SubExpr can only
be CXXMemberCallExpr.
rdar://problem/24364077
llvm-svn: 259591