Make Type and Attribute classes trivially copyable

This requires using explicitly default copy constructor and copy assignment
operator instead of hand-rolled ones. These classes are indeed cheap to copy
since they are wrappers around a pointer to the implementation. This change
makes sure templated code can use standard type traits to understand that
copying such objects is cheap and appeases analysis tools such as clang-tidy.

PiperOrigin-RevId: 286725565