gh-106320: Remove private AC converter functions (#108505)

Move these private functions to the internal C API
(pycore_abstract.h):

* _Py_convert_optional_to_ssize_t()
* _PyNumber_Index()

Argument Clinic now emits #include "pycore_abstract.h" when these
functions are used.

The parser of the c-analyzer tool now uses a list of files which use
the limited C API, rather than a list of files using the internal C
API.

Python/clinic/_warnings.c.h

@@ -7,6 +7,7 @@ preserve
 #  include "pycore_runtime.h"       // _Py_ID()
 #endif
 
+#include "pycore_abstract.h"       // _PyNumber_Index()
 
 PyDoc_STRVAR(warnings_warn__doc__,
 "warn($module, /, message, category=None, stacklevel=1, source=None, *,\n"
@@ -243,4 +244,4 @@ warnings_filters_mutated(PyObject *module, PyObject *Py_UNUSED(ignored))
 {
     return warnings_filters_mutated_impl(module);
 }
-/*[clinic end generated code: output=0e2b367a662bf51b input=a9049054013a1b77]*/
+/*[clinic end generated code: output=c8a6dc1403fba1d5 input=a9049054013a1b77]*/

Python/getargs.c

@@ -2,6 +2,7 @@
 /* New getargs implementation */
 
 #include "Python.h"
+#include "pycore_abstract.h"      // _PyNumber_Index()
 #include "pycore_dict.h"          // _PyDict_HasOnlyStringKeys()
 #include "pycore_pylifecycle.h"   // _PyArg_Fini
 #include "pycore_tuple.h"         // _PyTuple_ITEMS()