RFR: 8256955: Move includes of events.hpp out of header files

[Stefan Karlsson]

I was making changes to events.hpp and saw that the majority of cpp files were recompiled. There are two header files that include events.hpp. I propose that we remove those usages by:

jvmci.hpp: Use a forward declaration
collectedHeap.hpp: Move its usage to the cpp file

events.hpp also includes thread.hpp, which brings in a large portion of the HotSpot headers. So, this change is also good in that regard.

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Commit messages:
 - 8256955: Move includes of events.hpp out of header files

Changes: https://git.openjdk.java.net/jdk/pull/2059/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2059&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256955
  Stats: 58 lines in 7 files changed: 31 ins; 27 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2059.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2059/head:pull/2059

PR: https://git.openjdk.java.net/jdk/pull/2059