RFR: 8267043: IntelliJ project doesn't handle generated sources correctly [v2]

[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`).

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Bump source level

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4000/files
  - new: https://git.openjdk.java.net/jdk/pull/4000/files/acb05ab9..cf10eadc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4000&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4000&range=00-01

  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