c++: Improve contracts support in modules [PR108205]

Modules makes some assumptions about types that currently aren't
fulfilled by the types created in contracts logic.  This patch ensures
that exporting inline functions using contracts works again with
modules.

	PR c++/108205

gcc/cp/ChangeLog:

	* contracts.cc (get_pseudo_contract_violation_type): Give names
	to generated FIELD_DECLs.
	(declare_handle_contract_violation): Mark contract_violation
	type as external linkage.
	(build_contract_handler_call): Ensure any builtin declarations
	created here aren't treated as attached to the current module.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/contracts-5_a.C: New test.
	* g++.dg/modules/contracts-5_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>