[lworld] RFR: 8287241: Add IdentityException to report that a value object is not valid [v4]

[Roger Riggs]

> Add class java.lang.IdentityException, when thrown indicates that an identity object was required but a value object was supplied.
> 
> Also add methods `java.util.Objects.requireIdentity(obj)` and `Objects.requireIdentity(obj, message)` to check and throw.
> 
> Updated java.lang.ref.Reference and related tests to use new exception and method.

Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:

  Add IdentityException constructor with a single Throwable cause.

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/696/files
  - new: https://git.openjdk.org/valhalla/pull/696/files/0d9a92c6..92c6e90e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=valhalla&pr=696&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=valhalla&pr=696&range=02-03

  Stats: 16 lines in 1 file changed: 13 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/696.diff
  Fetch: git fetch https://git.openjdk.org/valhalla pull/696/head:pull/696

PR: https://git.openjdk.org/valhalla/pull/696