RFR: 8273439: Fix G1CollectedHeap includes and forward declarations

[Thomas Schatzl]

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

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Commit messages:
 - Fix forward declarations and includes

Changes: https://git.openjdk.java.net/jdk/pull/5393/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5393&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273439
  Stats: 38 lines in 1 file changed: 7 ins; 23 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5393.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5393/head:pull/5393

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