Mark discriminants as artificial in gdb.dwarf2/variant.exp

While working on a variant part patch, I notcied that
gdb.dwarf2/variant.exp does not mark the discriminant members as
DW_AT_artificial.  However, it should, as this is what the real Rust
compiler does, and how the Rust language support is supposed to work.

gdb/testsuite/ChangeLog
2020-03-11  Tom Tromey  <tromey@adacore.com>

	* gdb.dwarf2/variant.exp: Mark discriminants as artificial.