RFR: JDK-8282411: Add useful predicates to ElementKind

[Joe Darcy]

Discussion from a recent review in javadoc, adding some more functionality to ElementKind predicates.

Please also review the CSR: https://bugs.openjdk.java.net/browse/JDK-8282461

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Commit messages:
 - Add tests.
 - JDK-8282411: Add useful predicates to ElementKind

Changes: https://git.openjdk.java.net/jdk/pull/7637/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7637&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282411
  Stats: 103 lines in 2 files changed: 97 ins; 1 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7637.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7637/head:pull/7637

PR: https://git.openjdk.java.net/jdk/pull/7637