RFR: 8336588: Ensure Transform downstream receives upstream start items only after downstream started

[Chen Liang]

There's another bug in ClassFile transform composition where the downstream transform receives items from upstream transform's chained builders before the downstream transform itself starts. This is a simple fix, and a test case against `ClassTransform` is added.

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Commit messages:
 - 8336588: Ensure Transform downstream receives upstream start items only after downstream started

Changes: https://git.openjdk.org/jdk/pull/20227/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20227&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336588
  Stats: 63 lines in 2 files changed: 49 ins; 9 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/20227.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20227/head:pull/20227

PR: https://git.openjdk.org/jdk/pull/20227