RFR: 8290903: Enable function warning attribute for Clang build once Clang supports merging

[Afshin Zafari]

The warning attribute is enabled..

### Test
mach5 tier1-5.

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Commit messages:
 - 8290903: Enable function warning attribute for Clang build once Clang supports merging
 - 8290903: Enable function warning attribute for Clang build once Clang supports merging

Changes: https://git.openjdk.org/jdk/pull/12634/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12634&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290903
  Stats: 6 lines in 1 file changed: 0 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12634.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12634/head:pull/12634

PR: https://git.openjdk.org/jdk/pull/12634