Add support for DW_FORM_GNU_ref_alt and DW_FORM_GNU_strp_alt. (#176056)

LLDB's DWARF parser didn't support parsing DW_FORM_GNU_ref_alt and
DW_FORM_GNU_strp_alt forms which would cause any file loaded by LLDB to
fail to parse any DWARF. Added support for parsing this information
only, not for actually finding the debug info reference to an alternate
file or a string in an alternate file. These extensions are used by DWZ
files which are present in some linux distros, so it will be good for
LLDB to just be able to parse these without emitting an error like:

(lldb) b bar
warning: (arm64) /tmp/a.out unsupported DW_FORM values: 0x1f20 0x1f21
GitOrigin-RevId: 16149ca93227d0d26a2f7387e95ad30971e99f43