RFR: 8304937: BufferedFieldBuilder.Model missing writeTo(DirectClassBuilder)

[Chen Liang]

Please review this simple patch to Classfile API that fixes a missing override that otherwise affects usage of chained class transforms. A test is included, that it fails on the missing method without this patch.

Please review a few other patches fixing bugs affecting normal usage of the Classfile API as well:
#13167 #13021 #12996

I am trying to write a test for my in-progress patch to unify parameter mapping in core reflection in anticipation of #9862, however this bug prevents me from dropping method Signature and MethodParameters attributes with chained class transforms.

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Commit messages:
 - 8304937: BufferedFieldBuilder.Model missing writeTo(DirectClassBuilder)

Changes: https://git.openjdk.org/jdk/pull/13187/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13187&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304937
  Stats: 86 lines in 2 files changed: 85 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13187.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13187/head:pull/13187

PR: https://git.openjdk.org/jdk/pull/13187