General contracts impl cleanup

[JAChapmanII]

Use void_node for places we need a "NOP" instead of building a new node (build_contract_check in case of CCS_IGNORE, any other places?)

Ensure all remaining warning/error messages don't use %s to insert English text.

Move attr name checking fully into cp_parser_contract_attribute_spec, return NULL_TREE early when the contract being parsed isn't a contract.

In build_contract_functor_declaration there's the note:

  /* FIXME will later optimizations delete unused args to prevent extra arg
   * passing? do we care? */

about potentially generating pre/post fns with fewer arguments as a potential optimization. This should probably be downgraded to just a note about a possible optimization (maybe using the optimizer partial-inlining code) and not an actual FIXME.

Comment and rename build_contract_handler_fn, possibly build_contract_handler_fn_call.
Move setting current_function_returns_abnormally and TREE_NOTHROW into here. Are we always actually building this call from inside the function that needs these set? Use build_call_n?

These shouldn't be set here, they should be set as part of building the call to the handler. You should get that by changing build_call_expr to build_call_n...

Fix possibly outdated comment

  /* FIXME: It looks  like we have two bits of information for
     continuing.  Is this right?  */

I think this was referring to the fact that we have both continue_ and continuation_mode arguments on __on_contract_violation rather than needing to distinguish >2 states.

Elaborate about roles.

Move more code from decl.c into the contracts file cxx-contracts.c.

Rename cxx-contracts.c to contracts.cc.

Figure out where contract.cc in libstdc++-v3 should end up; does it need to be moved to libstdc++-v3/libsupc++?