Correctly compute length of DW_TAG_variant_part union

Correctly compute length of DW_TAG_variant_part union

Currently, gdb internally transforms DW_TAG_variant_part into a union
(with some special attbributes).  When doing so, it computes the
length of this union from the length of the fields.  However, this
computation didn't include the offset of these fields, resulting in
the length being too short.

This is not a problem given the way the code currently works.
However, I have a patch series to switch gdb to value-based printing,
where this does have an impact.

Tested on x86-64 Fedora 28; and, considering that this only affects
Rust, I am checking it in.

gdb/ChangeLog
2019-11-30  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (dwarf2_add_field): Include field offset when
	computing variant part length.

Change-Id: I25d84fc237eb3c1e7f11f6eaf35ffe198efde6cc