RFR: 8224922: Access JavaFileObject from Element(s) [v15]

[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 two additional commits since the last revision:

 - Merge pull request #6 from lahodaj/JDK-8224922-4
   
   Adding forgotten file/fixing test.
 - Adding forgotten file/fixing test.

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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/04e8ea4b..a62ff194

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=14
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=13-14

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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