RFR: Add generations to freeset [v20]

[Y. Srinivas Ramakrishna]

On Sat, 29 Apr 2023 13:45:09 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> Hmm, wait, the only non-public method is `clear_internal` which is anyway exposed via `clear_all`. 
>> 
>> In that case, is the friend accessing a private data member field? 
>> 
>> Or perhaps the friend annotation is now obsolete from an earlier round of development and must now be removed.
>
> You are correct that I no longer need the friend class.  In a previous version of the code, I had an assert on ShenandoahFreeSets::clear_all() that was not appropriate for all calls (in particular the initialization call) from ShenandoahFreeSet::clear_internal().  I'll make this change. (hmmm. Or should I put that assert_heaplocked()) back into ShenandoahFreeSets::clear_all()?)

ok.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/250#discussion_r1181698676