RFR: 8331081: 'internal proprietary API' diagnostics if --system is configured to an earlier JDK version [v2]

[Liam Miller-Cushon]

> This change fixes a bug preventing javac from emitting 'compiler.warn.sun.proprietary' diagnostics if `--system` is set to a non-default value. The diagnostics are currently emitted for values of `--release`, and for the default value of `--system`.

Liam Miller-Cushon has updated the pull request incrementally with one additional commit since the last revision:

  don't accumulate user modules in supplementaryFlags

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19069/files
  - new: https://git.openjdk.org/jdk/pull/19069/files/4cb9915e..fef11730

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19069&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19069&range=00-01

  Stats: 14 lines in 1 file changed: 5 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/19069.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19069/head:pull/19069

PR: https://git.openjdk.org/jdk/pull/19069