Fix a few DWARF bugs with -fgnat-encodings=minimal

Fix a few DWARF bugs with -fgnat-encodings=minimal

The -fgnat-encodings=minimal switch tells the compiler to generate mostly
pure DWARF for the GNAT compiler and it contains some bugs related to
discriminated record types with variant part.

	* dwarf2out.c (add_data_member_location_attribute): Account for
	the variant part offset in the computation of the data bit offset.
	(add_bit_offset_attribute): Remove CTX parameter.  Pass a new
	context in the call to field_byte_offset.
	(gen_field_die): Adjust call to add_bit_offset_attribute and
	remove confusing assertion.
	(analyze_variant_discr): Deal with boolean subtypes.