[clang] [CIR][RISCV][NFC] Add CIRGenBuiltinRISCV file to support RISCV builtins codegen (PR #186050)

[Andy Kaylor via cfe-commits]

================
@@ -0,0 +1,103 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This contains code to emit RISC-V Builtin calls as CIR or a function call
+// to be later resolved.
+//
+//===----------------------------------------------------------------------===//
+
+#include "CIRGenFunction.h"
+#include "clang/Basic/TargetBuiltins.h"
+
+using namespace clang;
+using namespace clang::CIRGen;
+
+std::optional<mlir::Value>
+CIRGenFunction::emitRISCVBuiltinExpr(unsigned builtinID, const CallExpr *e) {
+  switch (builtinID) {
+  default:
+    return std::nullopt;
----------------
andykaylor wrote:

```suggestion
  default: llvm_unreachable("unexpected builtin ID");
```
This is what the classic codegen version of this file does.

https://github.com/llvm/llvm-project/pull/186050