[clang] [C++26] [Contracts] Support Parser, AST and AST Serialization (1/n) (PR #205739)

[Yihan Wang via cfe-commits]

================
@@ -2012,6 +2003,74 @@ enum class MultiVersionKind {
   TargetVersion
 };
 
+/// Represents a single C++26 contract annotation (P2900R14).
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yronglin wrote:

```suggestion
/// Represents a single C++26 contract annotation.
```

https://github.com/llvm/llvm-project/pull/205739