Add analyzer plugin support and CPython GIL example

This patch adds a new GCC plugin event: PLUGIN_ANALYZER_INIT, called
when -fanalyzer is starting, allowing for GCC plugins to register
additional state-machine-based checks within -fanalyzer.  The idea
is that 3rd-party code might want to add domain-specific checks for
its own APIs - with the caveat that the analyzer is itself still
rather experimental.

As an example, the patch adds a proof-of-concept plugin to the testsuite
for checking CPython code: verifying that code that relinquishes
CPython's global interpreter lock doesn't attempt to do anything with
PyObjects in the sections where the lock isn't held.  It also adds a
warning about nested releases of the lock, which is forbidden.
For example:

demo.c: In function 'foo':
demo.c:11:3: warning: use of PyObject '*(obj)' without the GIL
   11 |   Py_INCREF (obj);
      |   ^~~~~~~~~
  'test': events 1-3
    |
    |   15 | void test (PyObject *obj)
    |      |      ^~~~
    |      |      |
    |      |      (1) entry to 'test'
    |   16 | {
    |   17 |   Py_BEGIN_ALLOW_THREADS
    |      |   ~~~~~~~~~~~~~~~~~~~~~~
    |      |   |
    |      |   (2) releasing the GIL here
    |   18 |   foo (obj);
    |      |   ~~~~~~~~~
    |      |   |
    |      |   (3) calling 'foo' from 'test'
    |
    +--> 'foo': events 4-5
           |
           |    9 | foo (PyObject *obj)
           |      | ^~~
           |      | |
           |      | (4) entry to 'foo'
           |   10 | {
           |   11 |   Py_INCREF (obj);
           |      |   ~~~~~~~~~
           |      |   |
           |      |   (5) PyObject '*(obj)' used here without the GIL
           |

Doing so requires adding some logic for ignoring macro expansions in
analyzer diagnostics, since the insides of Py_INCREF and
Py_BEGIN_ALLOW_THREADS are not of interest to the user for these cases.

gcc/analyzer/ChangeLog:
	* analyzer-pass.cc (pass_analyzer::execute): Move sorry call to...
	(sorry_no_analyzer): New.
	* analyzer.h (class state_machine): New forward decl.
	(class logger): New forward decl.
	(class plugin_analyzer_init_iface): New.
	(sorry_no_analyzer): New decl.
	* checker-path.cc (checker_path::fixup_locations): New.
	* checker-path.h (checker_event::set_location): New.
	(checker_path::fixup_locations): New decl.
	* diagnostic-manager.cc
	(diagnostic_manager::emit_saved_diagnostic): Call
	checker_path::fixup_locations, and call fixup_location
	on the primary location.
	* engine.cc: Include "plugin.h".
	(class plugin_analyzer_init_impl): New.
	(impl_run_checkers): Invoke PLUGIN_ANALYZER_INIT callbacks.
	* pending-diagnostic.h (pending_diagnostic::fixup_location): New
	vfunc.

gcc/ChangeLog:
	* doc/plugins.texi (Plugin callbacks): Add PLUGIN_ANALYZER_INIT.
	* plugin.c (register_callback): Likewise.
	(invoke_plugin_callbacks_full): Likewise.
	* plugin.def (PLUGIN_ANALYZER_INIT): New event.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/analyzer_gil_plugin.c: New test.
	* gcc.dg/plugin/gil-1.c: New test.
	* gcc.dg/plugin/gil.h: New header.
	* gcc.dg/plugin/plugin.exp (plugin_test_list): Add the new plugin
	and test.

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