[jdk16] RFR: 8257596: Clarify trusted final fields for record classes [v3]

[Mandy Chung]

> This is a follow-up on JDK-8255342 that removes non-specified JVM checks on classes with RecordComponents attributes.
> 
> See the discussion at https://mail.openjdk.java.net/pipermail/amber-spec-experts/2020-December/002670.html
> 
> That fixes trusting final fields of records to align with the JLS definition of a record class.  `InstanceKlass::is_record` is fixed to check a record class must be final and a direct subclass of `java.lang.Record` with the presence of the `Record` attribute. There is no change to JVM class file validation. I  also propose clearly define:
> - `JVM_IsRecord` returns true if the given class is a record i.e. final and direct subclass of `java.lang.Record` with `Record` attribute
> - `JVM_GetRecordComponents `returns an `RecordComponents` array if `Record` attribute is present; otherwise, returns NULL. This does not check if it's a record class or not. So it may return non-null on a non-record class if it has `Record` attribute. So `JVM_GetRecordComponents` returns the content of the classfile.
> 
> tier1-tier3 and jck-runtime:vm and jck-runtime:lang tests all passed.

Mandy Chung has updated the pull request incrementally with one additional commit since the last revision:

  Update comments in Class::getRecordComponents0

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Changes:
  - all: https://git.openjdk.java.net/jdk16/pull/14/files
  - new: https://git.openjdk.java.net/jdk16/pull/14/files/4ebee9b1..f0df17a6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk16&pr=14&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk16&pr=14&range=01-02

  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/14.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/14/head:pull/14

PR: https://git.openjdk.java.net/jdk16/pull/14