[clang] [CIR] Upstream proper function alias lowering (PR #150520)

[Henrich Lauko via cfe-commits]

================
@@ -1190,6 +1248,12 @@ mlir::LogicalResult CIRToLLVMFuncOpLowering::matchAndRewrite(
       resultType ? resultType : mlir::LLVM::LLVMVoidType::get(getContext()),
       signatureConversion.getConvertedTypes(),
       /*isVarArg=*/fnType.isVarArg());
+
+  // If this is an alias, it needs to be lowered to llvm::AliasOp.
+  std::optional<mlir::FlatSymbolRefAttr> aliasee = op.getAliaseeAttr();
+  if (aliasee && *aliasee)
+    return matchAndRewriteAlias(op, *aliasee, llvmFnTy, adaptor, rewriter);
----------------
xlauko wrote:

```suggestion
  if (std::optional<llvm::StringRef> aliasee = op.getAliasee())
    return matchAndRewriteAlias(op, *aliasee, llvmFnTy, adaptor, rewriter);
```

https://github.com/llvm/llvm-project/pull/150520