analyzer: implement reference count checking for CPython plugin [PR107646]

This patch introduces initial support for reference count checking of
PyObjects in relation to the Python/C API for the CPython plugin.
Additionally, the core analyzer underwent several modifications to
accommodate this feature. These include:

- Introducing support for callbacks at the end of
  region_model::pop_frame. This is our current point of validation for
  the reference count of PyObjects.
- An added optional custom stmt_finder parameter to
  region_model_context::warn. This aids in emitting a diagnostic
  concerning the reference count, especially when the stmt_finder is
  NULL, which is currently the case during region_model::pop_frame.

The current diagnostic we emit relating to the reference count
appears as follows:

rc3.c:23:10: warning: expected ‘item’ to have reference count: ‘1’ but
ob_refcnt field is: ‘2’
   23 |   return list;
      |          ^~~~
  ‘create_py_object’: events 1-4
    |
    |    4 |   PyObject* item = PyLong_FromLong(3);
    |      |                    ^~~~~~~~~~~~~~~~~~
    |      |                    |
    |      |                    (1) when ‘PyLong_FromLong’ succeeds
    |    5 |   PyObject* list = PyList_New(1);
    |      |                    ~~~~~~~~~~~~~
    |      |                    |
    |      |                    (2) when ‘PyList_New’ succeeds
    |......
    |   14 |   PyList_Append(list, item);
    |      |   ~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |   |
    |      |   (3) when ‘PyList_Append’ succeeds, moving buffer
    |......
    |   23 |   return list;
    |      |          ~~~~
    |      |          |
    |      |          (4) here
    |

This is a WIP in several ways:
- Currently, functions returning PyObject * are assumed to always
  produce a new reference.
- The validation of reference count is only for PyObjects created within
  a function body. Verifying reference counts for PyObjects passed as
  parameters is not supported in this patch.

gcc/analyzer/ChangeLog:
	PR analyzer/107646
	* engine.cc (impl_region_model_context::warn): New optional
	parameter.
	* exploded-graph.h (class impl_region_model_context): Likewise.
	* region-model.cc (region_model::pop_frame): New callback
	feature for region_model::pop_frame.
	* region-model.h (struct append_regions_cb_data): Likewise.
	(class region_model): Likewise.
	(class region_model_context): New optional parameter.
	(class region_model_context_decorator): Likewise.

gcc/testsuite/ChangeLog:
	PR analyzer/107646
	* gcc.dg/plugin/analyzer_cpython_plugin.c: Implements reference
	count checking for PyObjects.
	* gcc.dg/plugin/cpython-plugin-test-2.c: Moved to...
	* gcc.dg/plugin/cpython-plugin-test-PyList_Append.c: ...here
	(and added more tests).
	* gcc.dg/plugin/cpython-plugin-test-1.c: Moved to...
	* gcc.dg/plugin/cpython-plugin-test-no-Python-h.c: ...here (and
	added more tests).
	* gcc.dg/plugin/plugin.exp: New tests.
	* gcc.dg/plugin/cpython-plugin-test-PyList_New.c: New test.
	* gcc.dg/plugin/cpython-plugin-test-PyLong_FromLong.c: New test.

Signed-off-by: Eric Feng <ef2648@columbia.edu>

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