RFR: 8224922: Access JavaFileObject from Element(s) [v7]

[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.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Appease jcheck.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5038/files
  - new: https://git.openjdk.java.net/jdk/pull/5038/files/1584c4b4..50ce61b2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=05-06

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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