java.lang.Record should be declared with an explicit constructor

Summary

The type java.lang.Record should use an explicit constructor rather than a default one.

Problem

Relying on a default constructor is not prudent for platform classes.

Solution

Add an explicit protected constructor.

Specification

--- old/src/java.base/share/classes/java/lang/Record.java   2020-01-06 13:47:31.740871560 -0800
+++ new/src/java.base/share/classes/java/lang/Record.java   2020-01-06 13:47:31.552777567 -0800
@@ -91,6 +91,11 @@
                              essentialAPI=true)
 public abstract class Record {
     /**
+     * Constructor for record classes to call.
+     */
+    protected Record() {}
+
+    /**
      * Indicates whether some other object is "equal to" this one.  In addition
      * to the general contract of {@link Object#equals(Object)},
      * record classes must further participate in the invariant that when