Fix regression in variant part handling

Fix regression in variant part handling

My series to change DWARF attribute handling to be type-safe
introduced a regression in gdb.ada/variant.exp.  handle_variant was
using as_unsigned on an attribute with DW_FORM_sdata.  This patch
changes it to use constant_value instead.

2020-09-30  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.c (handle_variant): Use constant_value.