RFR: 8268148: unchecked warnings handle ? and ? extends Object differently

[Vicente Romero]

Please review this PR. Currently javac handles differently `?` and `? extends Object` when reporting unchecked warnings when according to the spec they should be handled the same way. This PR is fixing this.

TIA

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Commit messages:
 - 8268148: unchecked warnings handle ? and ? extends Object differently

Changes: https://git.openjdk.java.net/jdk/pull/5224/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5224&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268148
  Stats: 46 lines in 3 files changed: 43 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5224.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5224/head:pull/5224

PR: https://git.openjdk.java.net/jdk/pull/5224