[clang] [clang] Implement P3176R1: The Oxford variadic comma (PR #117524)

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@@ -0,0 +1,36 @@
+// RUN: %clang_cc1 -std=c++2c -fsyntax-only -verify %s
+
+void a(...);
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Sirraide wrote:

A few more test cases that came to mind:
```c++
struct S {
    void g(this S...) {}
};

template <typename ...Ts>
void f(Ts......) {}

template <typename T>
void g(T...) {}

```

https://github.com/llvm/llvm-project/pull/117524