analyzer: add .fpath.txt dumps to -fdump-analyzer-feasibility

I found this extension to -fdump-analyzer-feasibility very helpful when
debugging PR analyzer/105285.

gcc/analyzer/ChangeLog:
	* diagnostic-manager.cc (epath_finder::process_worklist_item):
	Call dump_feasible_path when a path that reaches the the target
	enode is found.
	(epath_finder::dump_feasible_path): New.
	* engine.cc (feasibility_state::dump_to_pp): New.
	* exploded-graph.h (feasibility_state::dump_to_pp): New decl.
	* feasible-graph.cc (feasible_graph::dump_feasible_path): New.
	* feasible-graph.h (feasible_graph::dump_feasible_path): New
	decls.
	* program-point.cc (function_point::print): Fix missing trailing
	newlines.
	* program-point.h (program_point::print_source_line): Remove
	unimplemented decl.

gcc/ChangeLog:
	* doc/invoke.texi (-fdump-analyzer-feasibility): Mention the
	fpath.txt output.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>