Add publish/subscribe topics and channel: pass_events

Add publish/subscribe topics and channel: pass_events

This patch adds a new "struct compiler_channels" to hold channels
relating to the compiler that plugins (or diagnostic sinks) might want
to subscribe to events for, accessed from the global gcc::context
object, along with a new gcc/topics/ source subdirectory to hold
strongly-typed publish/subscribe topics relating to the compiler.

For now, there is just one: pass_events_channel, which, if there are any
subscribers, issues notifications about passes starting/stopping on
particular functions, using topics::pass_events, declared in
topics/pass-events.h, but followup patches add more kinds of
notification channel.

A toy plugin in the testsuite shows how this could be used to build a
progress notification UI for the compiler, and a followup patch uses the
channel to (optionally) capture CFG information at each stage of
optimization in machine-readable form into a SARIF sink.

gcc/ChangeLog:
	* channels.h: New file.
	* context.cc: Define INCLUDE_LIST.  Include "channels.h".
	(gcc::context::context): Create m_channels.
	(gcc::context::~context): Delete it.
	* context.h (struct compiler_channels): New forward decl.
	(gcc::context::get_channels): New accessor.
	(gcc::context::m_channels): New field.
	* passes.cc: Define INCLUDE_LIST.  Include "topics/pass-events.h"
	and "channels.h".
	(execute_one_pass): If the global context's pass_events_channel
	has subscribers, publish before_pass and after_pass events to it.
	* topics/pass-events.h: New file.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/plugin.exp: Add progress_notifications_plugin.cc.
	* gcc.dg/plugin/progress_notifications_plugin.cc: New test plugin.

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