RFR: 8224922: Access JavaFileObject from Element(s)

[Joe Darcy]

Initial review to get some comments on the shape of the API. Tests are needed of course, as well as some tuning of the spec to better describe differences in behavior when class files rather than source files are the backing file type.

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Commit messages:
 - 8224922: Access JavaFileObject from Element(s)

Changes: https://git.openjdk.java.net/jdk/pull/5038/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8224922
  Stats: 74 lines in 2 files changed: 74 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5038.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5038/head:pull/5038

PR: https://git.openjdk.java.net/jdk/pull/5038