c++: build contract_violation object directly

c++: build contract_violation object directly

The __on_contract_violation function was an awkward library dependency;
instead, let's build up a temporary contract_violation and call
handle_contract_violation directly.

gcc/cp/ChangeLog:

	* cp-tree.h (enum cp_tree_index): Remove
	CPTI_ON_CONTRACT_VIOLATION*, add CPTI_PSEUDO_CONTRACT_VIOLATION.
	(pseudo_contract_violation_type): New.
	(on_contract_violation_fn)
	(on_contract_violation_never_fn): Remove.
	* contracts.cc (get_pseudo_contract_violation_type): New.
	(build_contract_violation): New.
	(declare_handle_contract_violation): New.
 	(build_contract_handler_call): Use them.
	(build_contract_check): Call terminate here.
	(init_contract_processing): Remove.
	* decl.cc (cxx_init_decl_processing): Don't call it.

libstdc++-v3/ChangeLog:

	* src/experimental/contract.cc (__on_contract_violation): Remove.
	* include/experimental/contract: Remove its comment.

gcc/testsuite/ChangeLog:

	* g++.dg/contracts/contracts14.C
	* g++.dg/contracts/contracts15.C: Remove __on_contract_violation.