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

[Yeting Kuo via llvm-commits]

================
@@ -2824,8 +2827,12 @@ bool RISCVAsmParser::parseDirectiveOption() {
         break;
       }
 
-      auto Ext = llvm::lower_bound(RISCVFeatureKV, Arch);
-      if (Ext == std::end(RISCVFeatureKV) || StringRef(Ext->Key) != Arch ||
+      std::string &&Feature = RISCVISAInfo::getTargetFeatureForExtension(Arch);
----------------
yetingk wrote:

I think it might less call a string constructor() and just use the return string value.

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