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

[Sirui Mu via cfe-commits]

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@@ -2369,13 +2393,15 @@ mlir::LogicalResult CIRToLLVMFuncOpLowering::matchAndRewrite(
          "expected single location or unknown location here");
 
   mlir::LLVM::Linkage linkage = convertLinkage(op.getLinkage());
-  assert(!cir::MissingFeatures::opFuncCallingConv());
-  mlir::LLVM::CConv cconv = mlir::LLVM::CConv::C;
+  auto cconv = convertCallingConv(op.getCallingConv());
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Lancern wrote:

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

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