Check whether we can fully parse contract conditions earlier

[JAChapmanII]

We parse contract conditions on member functions during cp_parser_late_parsing_for_member when the body and default arguments are parsed since we need access to the completed class's members.

For defining declarations on deduced return type functions using the return value we may also need to defer contract parsing until the function body is completed so the return type can be deduced. Can we move this up in general for free functions?

[asutton]

Preconditions and postconditions can generally be parsed where they're written except in the following cases:

• On member functions, they must be parsed in the complete class context
• If the function has a deduced return type, special handling is required.

We deferred parsing postconditions to make the identifier bind directly to the DECL_RESULT. As an alternative, we could synthesize a fake parameter whose type is the return type of the function (or auto or decltype(auto)) and parse the postcondition in that context. For deduced return types, we would instantiate both the fake parm and the condition and replace the postcondition.