Fix bit offset regression

The type-safe attribute patch introduced a regression that can occur
when the DW_AT_bit_offset value is negative.  This can happen with
some Ada programs.

This patch fixes the problem.  It also fixes a minor oddity in the
existing scalar storage test -- this test was intended to assign a
smaller number of bits to the field.

2020-10-09  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.c (dwarf2_add_field): Handle signed offsets.

gdb/testsuite/ChangeLog
2020-10-09  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/scalar_storage/storage.adb (Another_Range): New type.
	(Rec): Add field.  Fix range.
	* gdb.ada/scalar_storage.exp: Update.