RFR: 8266846: Add java.time.InstantSource

[Stephen Colebourne]

8266846: Add java.time.InstantSource

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Commit messages:
 - 8266846: Add java.time.InstantSource
 - 8266846: Add java.time.InstantSource

Changes: https://git.openjdk.java.net/jdk/pull/4016/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4016&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266846
  Stats: 615 lines in 4 files changed: 519 ins; 83 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4016.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4016/head:pull/4016

PR: https://git.openjdk.java.net/jdk/pull/4016