Attempts to dump via -fopt-info from a plugin pass fail, due
to the dfi->alt_state for such passes never being set.

This is because the -fopt-info options were being set up per-pass
during option-parsing (via gcc::dump_manager::opt_info_enable_passes),
but this data was not retained or used it for passes created later
(for plugins and target-specific passes).

This patch fixes the issue by storing the -fopt-info options into
gcc::dump_manager, refactoring the dfi-setup code out of
opt_info_enable_passes, and reusing it for such passes, fixing the
issue.  The patch adds a demo plugin to test that dumping from a
plugin works.

gcc/ChangeLog:
	* dumpfile.c (gcc::dump_manager::dump_manager): Initialize new
	fields.
	(gcc::dump_manager::~dump_manager): Free m_optinfo_filename.
	(gcc::dump_manager::register_pass): New member function, adapted
	from loop body in gcc::pass_manager::register_pass, adding a
	call to update_dfi_for_opt_info.
	(gcc::dump_manager::opt_info_enable_passes): Store the
	-fopt-info options into the new fields.  Move the loop
	bodies into...
	(gcc::dump_manager::update_dfi_for_opt_info): ...this new member
	function.
	* dumpfile.h (struct opt_pass): New forward decl.
	(gcc::dump_manager::register_pass): New decl.
	(gcc::dump_manager::update_dfi_for_opt_info): New decl.
	(class gcc::dump_manager): Add fields "m_optgroup_flags",
	"m_optinfo_flags", and "m_optinfo_filename".
	* passes.c (gcc::pass_manager::register_pass): Move all of the
	dump-handling code to gcc::dump_manager::register_pass.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/dump-1.c: New test.
	* gcc.dg/plugin/dump_plugin.c: New test plugin.
	* gcc.dg/plugin/plugin.exp (plugin_test_list): Add the above.

From-SVN: r264844