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

[Afshin Zafari]

On Sat, 18 Feb 2023 15:26:04 GMT, Afshin Zafari <duke at openjdk.org> wrote:

> The warning attribute is enabled..
> 
> ### Test
> mach5 tier1-5.

> I'm a bit confused about the status here. Looking at:
> 
> [llvm/llvm-project#56519](https://github.com/llvm/llvm-project/issues/56519)
> 
> the issue is still open, not fixed. And the issue is reported as existing in version 14 so the uncommented version check seems wrong anyway. ??

The Fix Version of this issue is 21, so I mistakenly thought it is time for implementing the required changes.
I had to double check it first. Thanks for your time.

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