Fix handling of DW_AT_data_bit_offset

A newer version of GCC will now emit member locations using just
DW_AT_data_bit_offset, like:

 <3><14fe>: Abbrev Number: 1 (DW_TAG_member)
    <14ff>   DW_AT_name        : (indirect string, offset: 0x215e): nb_bytes
    <1503>   DW_AT_decl_file   : 1
    <1503>   DW_AT_decl_line   : 10
    <1504>   DW_AT_decl_column : 7
    <1505>   DW_AT_type        : <0x150b>
    <1509>   DW_AT_bit_size    : 31
    <150a>   DW_AT_data_bit_offset: 64

whereas earlier versions would emit something like:

 <3><164f>: Abbrev Number: 7 (DW_TAG_member)
    <1650>   DW_AT_name        : (indirect string, offset: 0x218d): nb_bytes
    <1654>   DW_AT_decl_file   : 1
    <1655>   DW_AT_decl_line   : 10
    <1656>   DW_AT_decl_column : 7
    <1657>   DW_AT_type        : <0x165f>
    <165b>   DW_AT_byte_size   : 4
    <165c>   DW_AT_bit_size    : 31
    <165d>   DW_AT_bit_offset  : 1
    <165e>   DW_AT_data_member_location: 8

That is, DW_AT_data_member_location is not emitted any more.  This is
a change due to the switch to DWARF 5 by default.

This change pointed out an existing bug in gdb, namely that the
attr_to_dynamic_prop depends on the presence of
DW_AT_data_member_location.  This patch moves the handling of
DW_AT_data_bit_offset into handle_data_member_location, and updates
attr_to_dynamic_prop to handle this new case.

A new test case is included.  This test fails with GCC 11, but passes
with an earlier version of GCC.