src: do not redefine private for GenDebugSymbols

Redefining private breaks any private inheritance in the
included files. We can simply declare GenDebugSymbols()
as friends in related classes to gain the access that we need.

Backport-PR-URL: https://github.com/nodejs/node/pull/19176
PR-URL: https://github.com/nodejs/node/pull/18653


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>