RFR: 8267043: IntelliJ project doesn't handle generated sources correctly

[Maurizio Cimadamore]

Since generated sources are placed in the build folder, and since the build folder is indicated by the IntelliJ project settings as "project output", IntelliJ indexing blissfully ignores all the classes which are generated during the build.
This simple patch fixes that, by simply pointing the project output to a non-existing folder inside the build path (called `idea`).

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Commit messages:
 - Redirect IntelliJ output folder to non-existent folder

Changes: https://git.openjdk.java.net/jdk/pull/4000/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4000&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267043
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4000.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4000/head:pull/4000

PR: https://git.openjdk.java.net/jdk/pull/4000