RFR: 8273439: Fix G1CollectedHeap includes and forward declarations

[Thomas Schatzl]

On Wed, 8 Sep 2021 23:46:14 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Hi all,
>> 
>>   can I have reviews for this change that
>>   * removes unnecessary includes (just one actually)
>>   * removes unnecessary forwards (either not used, or already covered by the #include declaration)
>>   * sorts the forward declarations
>>   * adds necessary forwards that were not directly included
>> 
>> Testing: building on various platforms
>> 
>> Thanks,
>>   Thomas
>
> Looks good.

Thanks @kimbarrett @albertnetymk for your reviews.

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PR: https://git.openjdk.java.net/jdk/pull/5393