c++, contracts: Add a contract scope per basic.scope.contract.

c++, contracts: Add a contract scope per basic.scope.contract.

This adds a scope for contract assertions as per the standard section
referenced.  Split out here because it is the only part of the implementation
that touches the name lookup files.

gcc/cp/ChangeLog:

	* cp-tree.h (processing_contract_condition): New.
	* name-lookup.cc (cp_binding_level_descriptor): Add contract scope.
	(begin_scope): Likewise.
	* name-lookup.h (enum scope_kind): Add sk_contract.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>