New API entrypoint: gcc_jit_function_dump_to_dot

gcc/jit/
	* libgccjit.h (gcc_jit_function_dump_to_dot): New.
	* libgccjit.map (gcc_jit_function_dump_to_dot): New.
	* libgccjit++.h (gccjit::function::dump_to_dot): New.
	* libgccjit.c (gcc_jit_function_dump_to_dot): New.
	* internal-api.h (gcc::jit::recording::function::dump_to_dot): New.
	(gcc::jit::recording::block::block): Add m_index member.
	(gcc::jit::recording::block::dump_to_dot): New.
	(gcc::jit::recording::block::dump_edges_to_dot): New.
	* internal-api.c (gcc::jit::recording::function::new_block): Give
	each block an index.
	(gcc::jit::recording::function::dump_to_dot): New.
	(gcc::jit::recording::block::dump_to_dot): New.
	(gcc::jit::recording::block::dump_edges_to_dot): New.