gh-113317: Add Codegen class to Argument Clinic (#117626)

gh-113317: Add Codegen class to Argument Clinic (#117626)

* Move ifndef_symbols, includes and add_include() from Clinic to
  Codegen. Add a 'codegen' (Codegen) attribute to Clinic.
* Remove libclinic.crenderdata module: move code to libclinic.codegen.
* BlockPrinter.print_block(): remove unused 'limited_capi' argument.
  Remove also 'core_includes' parameter.
* Add get_includes() methods.
* Make Codegen.ifndef_symbols private.
* Make Codegen.includes private.
* Make CConverter.includes private.