Fix PR12616 - gdb does not implement DW_AT_data_bit_offset

Fix PR12616 - gdb does not implement DW_AT_data_bit_offset

The DW_AT_data_bit_offset attribute was introduced by DWARF V4 and
allows specifying the offset of a data member within its containing
entity.  But although the new attribute was intended to replace
DW_AT_bit_offset for this purpose, GDB ignores it, and thus GCC still
emits DW_AT_bit_offset instead.  See also
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71669.

This change fixes GDB's lack of support for DW_AT_data_bit_offset and
adds an appropriate test case.

gdb/ChangeLog:

	PR gdb/12616
	* dwarf2read.c (dwarf2_add_field): Handle the DWARF V4 attribute
	DW_AT_data_bit_offset.

gdb/testsuite/ChangeLog:

	PR gdb/12616
	* gdb.dwarf2/nonvar-access.exp: New testcase.  Check that GDB
	respects the DW_AT_data_bit_offset attribute.