RFR: JDK-8318626: GetClassFields does not filter out ConstantPool.constantPoolOop field

[Alex Menkov]

FilteredFieldStream is intended to filter out some fields which does not represent valid java objects.
Currently the only filtered field is "constantPoolOop" from jdk.internal.reflect.ConstantPool class.
The change fixes FilteredFieldStream implementation to handle cases when filtered fields is the last field of the class ("constantPoolOop" is the only field of jdk.internal.reflect.ConstantPool)

Testing:
  - new test added that compares results of GetClassFields JVMTI function (it uses FilteredFieldStream) with Class.getDeclaredFields();
  - test/hotspot/jtreg/vmTestbase/nsk/jvmti/GetClassFields tests

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Commit messages:
 - FilteredFieldStream

Changes: https://git.openjdk.org/jdk/pull/16328/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16328&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318626
  Stats: 158 lines in 3 files changed: 153 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16328.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16328/head:pull/16328

PR: https://git.openjdk.org/jdk/pull/16328