Add new variant part code

Add new variant part code

This patch adds the infrastructure for the new variant part code.  At
this point, nothing uses this code.  This is done in a separate patch
to make it simpler to review.

I examined a few possible approaches to handling variant parts.  In
particular, I considered having a DWARF variant part be a union
(similar to how the Rust code works now); and I considered having type
fields have a flag indicating that they are variants.

Having separate types seemed bad conceptually, because these variants
aren't truly separate -- they rely on the "parent" type.  And,
changing how fields worked seemed excessively invasive.

So, in the end I thought the approach taken in this patch was both
simple to implement and understand, without losing generality.  The
idea in this patch is that all the fields of a type with variant parts
will be stored in a single field array, just as if they'd all been
listed directly.  Then, the variants are attached as a dynamic
property.  These control which fields end up in the type that's
constructed during dynamic type resolution.

gdb/ChangeLog
2020-04-24  Tom Tromey  <tromey@adacore.com>

	* gdbtypes.c (is_dynamic_type_internal): Check for variant parts.
	(variant::matches, compute_variant_fields_recurse)
	(compute_variant_fields_inner, compute_variant_fields): New
	functions.
	(resolve_dynamic_struct): Check for DYN_PROP_VARIANT_PARTS.
	Use resolved_type after type is made.
	(operator==): Add new cases.
	* gdbtypes.h (TYPE_HAS_VARIANT_PARTS): New macro.
	(struct discriminant_range, struct variant, struct variant_part):
	New.
	(union dynamic_prop_data) <variant_parts, original_type>: New
	members.
	(enum dynamic_prop_node_kind) <DYN_PROP_VARIANT_PARTS>: New constant.
	(enum dynamic_prop_kind) <PROP_TYPE, PROP_VARIANT_PARTS>: New
	constants.
	* value.c (unpack_bits_as_long): Now public.
	* value.h (unpack_bits_as_long): Declare.