Set TYPE_LENGTH on a variant part

Set TYPE_LENGTH on a variant part

gdb represents a DW_TAG_variant_part as a union.  While normally DWARF
would not set the size of a DW_TAG_variant_part, gdb's representation
requires the TYPE_LENGTH to be set.

This patch arranges to set the TYPE_LENGTH of a variant part if it has
not already been set.

This fixes some Rust regressions when testing against a version of
rustc that emits DW_TAG_variant_part.

gdb/ChangeLog
2018-08-31  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (dwarf2_add_field): Set the TYPE_LENGTH of the
	variant part type.