jit: don't use &vect[0] in libgccjit++.h [PR121516]

jit: don't use &vect[0] in libgccjit++.h [PR121516]

gcc/jit/ChangeLog:
	PR jit/121516
	* libgccjit++.h (context::new_struct_type): Replace use of
	&fields[0] with fields.data ().
	(context::new_function): Likewise for params.
	(context::new_rvalue): Likewise for elements.
	(context::new_call): Likewise for args.
	(block::end_with_switch): Likewise for cases.
	(block::end_with_extended_asm_goto): Likewise for goto_blocks.
	(context::new_struct_ctor): Likewise for fields and values.
	(context::new_array_ctor): Likewise for values.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>