RFR: JDK-8289249: Add methods to Elements for record constructors

[Joe Darcy]

Please review the addition of two predicates to better support records in the javax.lang.model API.

Please also review the CSR https://bugs.openjdk.org/browse/JDK-8291594

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Commit messages:
 - Appease jcheck.
 - Respond to CSR review feedback.
 - Include regression tests.
 - JDK-8289249: Add methods to Elements for record constructors

Changes: https://git.openjdk.org/jdk/pull/9694/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9694&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289249
  Stats: 224 lines in 3 files changed: 224 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9694.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9694/head:pull/9694

PR: https://git.openjdk.org/jdk/pull/9694