7bb441c86678 added the fixed-size string and address index forms
0x25 to 0x2c as DW_FORM_ref_strx1..4 and DW_FORM_ref_addrx1..4; the
standard (DWARF 5, table 7.6) names them DW_FORM_strx1..4 and
DW_FORM_addrx1..4, with no "ref". Rename them, keep the old spellings
as aliases since the header is installed, and update dwarf_dump.c's
form names accordingly. Add DW_LANG_C17 (0x2c) and DW_LANG_C23 (0x3e),
which clang emits for -std=c17 and -std=c23 units. ctfconv(1) needs
all of these; it carried them in a private header until now.
MFC after: 1 week