`decltype` expression is always `cpp_unexposed_expression`

[foonathan]

• cppast version: latest one
• parser: libclang_parser
• clang version: 3.9.1+

The expression inside a decltype() will always be a cpp_unexposed_expression and the type of the expression is not exposed either. On a related note, cpp_decltype_auto_type will never be created, decltype(auto) will create a simple cpp_auto_type instead.