[clang][RISCV] Update vcpop.v C interface to follow the nameing convention (#94318)

We named the intrinsics by replacing "." by "_" in the instruction
conventionally, so the `vcpopv_v` where the corresponding instruction is
`vcpop.v` should be named `vcpop_v`.

Author: 4vtomat

clang/include/clang/Basic/riscv_vector.td

@@ -2637,7 +2637,8 @@ let UnMaskedPolicyScheme = HasPassthruOperand in {
     defm vbrev   : RVVOutBuiltinSetZvbb;
     defm vclz    : RVVOutBuiltinSetZvbb;
     defm vctz    : RVVOutBuiltinSetZvbb;
-    defm vcpopv  : RVVOutBuiltinSetZvbb;
+    let IRName = "vcpopv", MaskedIRName = "vcpopv_mask" in
+    defm vcpop   : RVVOutBuiltinSetZvbb;
     let OverloadedName = "vwsll" in
     defm vwsll   : RVVSignedWidenBinBuiltinSetVwsll;
   }