[clang] [llvm] [RISCV] Teach .option arch to support experimental extensions. (PR #89727)

[Yeting Kuo via cfe-commits]

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@@ -51,6 +51,9 @@ STATISTIC(RISCVNumInstrsCompressed,
 
 static cl::opt<bool> AddBuildAttributes("riscv-add-build-attributes",
                                         cl::init(false));
+static cl::opt<bool>
+    DisableExperimentalExtension("riscv-disable-experimental-ext",
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yetingk wrote:

Since only clang needs specific option to allows experimental extensions.

https://github.com/llvm/llvm-project/pull/89727