RFR: 8339156: Use more fine-granular clang unused warnings

[Magnus Ihse Bursie]

Currently, we issue -Wno-unused for all files in clang, which is a rather big sledgehammer to get rid of some warnings that proliferate in a few areas of the build.

We should instead leave -Wunused turned on (as done by -Wall) and use a much more fine-grained approach to disabling specific warnings in specific files or libraries.

This is similar to what has been done for gcc in JDK-8339120.

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Commit messages:
 - 8339156: Use more fine-granular clang unused warnings

Changes: https://git.openjdk.org/jdk/pull/20770/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20770&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339156
  Stats: 60 lines in 16 files changed: 42 ins; 1 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/20770.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20770/head:pull/20770

PR: https://git.openjdk.org/jdk/pull/20770