analyzer: add -fno-analyzer-feasibility

This patch provides a new option "-fno-analyzer-feasibility" as a way
to disable feasibility-checking of the constraints along the control
flow paths for -fanalyzer diagnostics.  I'm adding this in the hope of
making it easier to debug issues involving the feasibility-checking
logic.

The patch adds a new rejected_constraint object which is captured if
exploded_path::feasible_p fails, and adds logic that uses this to emit
an additional custom_event within the checker_path for the diagnostic,
showing where in the control flow path the diagnostic would have been
rejected, and giving details of why.

gcc/analyzer/ChangeLog:
	* analyzer.h (struct rejected_constraint): New decl.
	* analyzer.opt (fanalyzer-feasibility): New option.
	* diagnostic-manager.cc (path_builder::path_builder): Add
	"problem" param and use it to initialize new field.
	(path_builder::get_feasibility_problem): New accessor.
	(path_builder::m_feasibility_problem): New field.
	(dedupe_winners::add): Remove inversion of logic in "if" clause,
	swapping if/else suites.  In the !feasible_p suite, inspect
	flag_analyzer_feasibility and add code to handle when this
	is off, accepting the infeasible path, but recording the
	feasibility_problem.
	(diagnostic_manager::emit_saved_diagnostic): Pass the
	feasibility_problem to the path_builder.
	(diagnostic_manager::add_events_for_eedge): If we have
	a feasibility_problem at this edge, use it to add a custom event.
	* engine.cc (exploded_path::feasible_p): Pass a
	rejected_constraint ** to model.maybe_update_for_edge and transfer
	ownership of any created instance to any feasibility_problem.
	(feasibility_problem::dump_to_pp): New.
	* exploded-graph.h (feasibility_problem::feasibility_problem):
	Drop "model" param; add rejected_constraint * param.
	(feasibility_problem::~feasibility_problem): New.
	(feasibility_problem::dump_to_pp): New decl.
	(feasibility_problem::m_model): Drop field.
	(feasibility_problem::m_rc): New field.
	* program-point.cc (function_point::get_location): Handle
	PK_BEFORE_SUPERNODE and PK_AFTER_SUPERNODE.
	* program-state.cc (program_state::on_edge): Pass NULL to new
	param of region_model::maybe_update_for_edge.
	* region-model.cc (region_model::add_constraint): New overload
	adding a rejected_constraint ** param.
	(region_model::maybe_update_for_edge): Add rejected_constraint **
	param and pass it to the various apply_constraints_for_ calls.
	(region_model::apply_constraints_for_gcond): Add
	rejected_constraint ** param and pass it to add_constraint calls.
	(region_model::apply_constraints_for_gswitch): Likewise.
	(region_model::apply_constraints_for_exception): Likewise.
	(rejected_constraint::dump_to_pp): New.
	* region-model.h (region_model::maybe_update_for_edge):
	Add rejected_constraint ** param.
	(region_model::add_constraint): New overload adding a
	rejected_constraint ** param.
	(region_model::apply_constraints_for_gcond): Add
	rejected_constraint ** param.
	(region_model::apply_constraints_for_gswitch): Likewise.
	(region_model::apply_constraints_for_exception): Likewise.
	(struct rejected_constraint): New.

gcc/ChangeLog:
	* doc/analyzer.texi (Analyzer Paths): Add note about
	-fno-analyzer-feasibility.
	* doc/invoke.texi (Static Analyzer Options): Add
	-fno-analyzer-feasibility.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/feasibility-2.c: New test.

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