RFR: 8266436: Synthetic constructor trees have non-null return type

[Jan Lahoda]

When javac generates the AST for default (synthetic) constructor, it will create a return type `void` for them, while for ordinary constructors, it uses `null` as the return type. This change proposes to consistently use `null` as the return type of constructors. (Eventually, we might want to avoid generating the synthetic constructor tree.)

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Commit messages:
 - 8266436: Synthetic constructor trees have non-null return type

Changes: https://git.openjdk.java.net/jdk/pull/3842/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3842&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266436
  Stats: 23 lines in 2 files changed: 17 ins; 3 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3842.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3842/head:pull/3842

PR: https://git.openjdk.java.net/jdk/pull/3842