Use std::vector for field lists in dwarf2read.c

Use std::vector for field lists in dwarf2read.c

This changes dwarf2read.c to use std::vector rather than a linked list
when managing the fields and base classes to be added to a type.  This
removes some bookkeeping types and also allows the removal of some
cleanups.

gdb/ChangeLog
2018-03-11  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (struct nextfield): Add initializers.
	(struct nextfnfield): Remove.
	(struct fnfieldlist): Add initializers.  Remove "length" and
	"head", use std::vector.
	(struct decl_field_list): Remove.
	(struct field_info): Add initializers.
	<fields, baseclasses>: Now std::vector.
	<nbaseclasses, nfnfields, typedef_field_list_count,
	nested_types_list_count>: Remove.
	(dwarf2_add_field, dwarf2_add_type_defn)
	(dwarf2_attach_fields_to_type, dwarf2_add_member_fn)
	(dwarf2_attach_fn_fields_to_type, handle_struct_member_die)
	(process_structure_scope): Update.