[clang] [CIR] Upstream support for function/call calling conventions (PR #181170)

[Sirui Mu via cfe-commits]

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@@ -1840,7 +1862,10 @@ rewriteCallOrInvoke(mlir::Operation *op, mlir::ValueRange callOperands,
   if (converter->convertTypes(cirResults, llvmResults).failed())
     return mlir::failure();
 
-  assert(!cir::MissingFeatures::opCallCallConv());
+  auto cconv = convertCallingConv(call.getCallingConv());
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Lancern wrote:

```suggestion
  std::optional<mlir::LLVM::CConv> cconv = convertCallingConv(call.getCallingConv());
```

https://github.com/llvm/llvm-project/pull/181170