[clang] [Clang] Cache stable DeclContext relationships (NFC) (PR #223047)

[Yanzuo Liu via cfe-commits]

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@@ -2101,6 +2102,17 @@ class DeclContext {
   /// another pointer.
   mutable Decl *LastDecl = nullptr;
 
+  /// A primary context whose identity cannot change. Incomplete C++ records
+  /// and contexts with mutable primary identities remain uncached.
+  /// Relaxed atomics permit cache population during concurrent read-only AST
+  /// traversal; they do not synchronize mutations to the AST itself.
+  mutable std::atomic<DeclContext *> CachedPrimaryContext = nullptr;
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zwuis wrote:

Do/Should we support concurrent traversal?

https://github.com/llvm/llvm-project/pull/223047