RFR: JDK-8255464: Cannot access ModuleTree in a CompilationUnitTree

[Jonathan Gibbons]

Please review a small change to add an overdue missing method to access the `ModuleTree` when a `CompilationUnitTree` represents a modular compilation unit.

CSR is in progress.

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Commit messages:
 - JDK-8255464: Cannot access ModuleTree in a CompilationUnitTree

Changes: https://git.openjdk.java.net/jdk/pull/2155/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2155&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255464
  Stats: 179 lines in 5 files changed: 171 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2155.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2155/head:pull/2155

PR: https://git.openjdk.java.net/jdk/pull/2155