[RISCV] Remove 'experimental-' from extension name in diagnostic message.

I'm not sure how to test this because the intrinsic availability
already seems to check this.

Author: topperc

clang/lib/Sema/SemaChecking.cpp

@@ -5323,9 +5323,9 @@ bool Sema::CheckRISCVBuiltinFunctionCall(const TargetInfo &TI,
     QualType Op3Type = TheCall->getArg(2)->getType();
     uint64_t ElemSize = Op1Type->isRVVType(32, false) ? 32 : 64;
     if (ElemSize == 64 && !TI.hasFeature("experimental-zvknhb"))
-      return
-          Diag(TheCall->getBeginLoc(), diag::err_riscv_type_requires_extension)
-              << Op1Type << "experimental-zvknhb";
+      return Diag(TheCall->getBeginLoc(),
+                  diag::err_riscv_type_requires_extension)
+             << Op1Type << "zvknhb";
 
     return CheckInvalidVLENandLMUL(TI, TheCall, *this, Op1Type, ElemSize << 2) ||
            CheckInvalidVLENandLMUL(TI, TheCall, *this, Op2Type, ElemSize << 2) ||