RFR: 8255560: Class::isRecord should check that the current class is final and not abstract

[Chris Hegarty]

Update Class::isRecord to only return true for classes that are final.

The removal of non-specified JVM checks on classes with a Record Attribute (see JDK-8255342), has resulted in more types of loadable classes that may contain a Record Attribute. Since these checks are not performed by the JVM anymore, Class::isRecord, and by extension Class::getRecordComponents, may return true or component values, respectively, for classes that are not well-formed record classes (as per the JLS), .e.g. non-final or abstract classes, that contain a record Attribute.

Core Reflection, Class::isRecord, already asserts checks that the JVM does not, e.g. that the direct superclass is java.lang.Record. Some points from the Java Language Specification for record classes:

 1. It is a compile-time error if a record declaration has the modifier abstract.
 2. A record declaration is implicitly final.
 3. The direct superclass type of a record class is Record.

Class::isRecord already ensures no.3. This issue proposes to add explicit checks in Core Reflection to ensure no.1 and no.2, since the JVM now allows such classes that contain a Record Attribute to be loaded.

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Commit messages:
 - Initial changes for 8255560

Changes: https://git.openjdk.java.net/jdk/pull/1543/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1543&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255560
  Stats: 241 lines in 5 files changed: 209 ins; 12 del; 20 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1543.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1543/head:pull/1543

PR: https://git.openjdk.java.net/jdk/pull/1543