[clang] [Clang] Fix crash on malformed std::partial_ordering static members (PR #172001)

[Oliver Hunt via cfe-commits]

================
@@ -67,3 +67,28 @@ struct comparable_t {
                                                                   expected-note {{defaulted 'operator<=>' is implicitly deleted because defaulted comparison of vector types is not supported}}
 };
 } // namespace GH137452
+
+namespace GH170015 {
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ojhunt wrote:

Would you mind adding an additional test - it will require a new file alas - to cover a few other cases:

```cpp
namespace std {
   using partial_ordering = int;
}
```

```cpp
namespace std {
   struct partial_ordering;
}
```

This last one just a paranoia check

```cpp
namespace std {
   template <class> struct `partial_ordering` {
    static const partial_ordering less;
    static const partial_ordering equivalent;
    static const partial_ordering greater;
    static const partial_ordering unordered;
  };
}
```

My approach to the test would be a single file with multiple invocations with -DTEST1, -DTEST2, .. etc  and use that to select the definition being tested.

https://github.com/llvm/llvm-project/pull/172001