analyzer: reimplement on_transition in terms of get_state/set_next_state

analyzer: reimplement on_transition in terms of get_state/set_next_state

This patch is further preliminary work towards generalizing sm-malloc.cc
beyond just malloc/free.

Reimplement sm_context's on_transition vfunc in terms of new get_state
and set_next_state vfuncs, so that in followup patches we can implement
richer transitions (e.g. where the states are parametrized by
allocator).

gcc/analyzer/ChangeLog:
	* diagnostic-manager.cc
	(null_assignment_sm_context::null_assignment_sm_context): Add old_state
	and ext_state params, initializing m_old_state and m_ext_state.
	(null_assignment_sm_context::on_transition): Split into...
	(null_assignment_sm_context::get_state): ...this new vfunc
	implementation and...
	(null_assignment_sm_context::set_next_state): ...this new vfunc
	implementation.
	(null_assignment_sm_context::m_old_state): New field.
	(null_assignment_sm_context::m_ext_state): New field.
	(diagnostic_manager::add_events_for_eedge): Pass in old state and
	ext_state when creating sm_ctxt.
	* engine.cc (impl_sm_context::on_transition): Split into...
	(impl_sm_context::get_state): ...this new vfunc
	implementation and...
	(impl_sm_context::set_next_state): ...this new vfunc
	implementation.
	* sm.h (sm_context::get_state): New pure virtual function.
	(sm_context::set_next_state): Likewise.
	(sm_context::on_transition): Convert from a pure virtual function
	to a regular function implemented in terms of get_state and
	set_next_state.