More changes to complete the dynamic type support for Stmt/Expr.

This set of changes includes:
- convert ExprCXX.h and add to the Xcode project file.
-- required adding CXXBoolLiteralExpr to StmtNodes.def.
-- required adding visitor support (decl/defn).
- make the class codes in StmtNodes.def explicit (to enable range checking).
-- this required changing all clients of the STMT macro.
- declare the instance data const.

llvm-svn: 39344