[clang] [APINotes] Upstream APINotesWriter (PR #65187)

[Egor Zhdan via cfe-commits]

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@@ -749,6 +750,17 @@ struct Context {
 
   Context(ContextID id, ContextKind kind) : id(id), kind(kind) {}
 };
+
+/// A temporary reference to an Objective-C selector, suitable for
+/// referencing selector data on the stack.
+///
+/// Instances of this struct do not store references to any of the
+/// data they contain; it is up to the user to ensure that the data
+/// referenced by the identifier list persists.
+struct ObjCSelectorRef {
+  unsigned NumPieces;
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egorzhdan wrote:

Good point, we're not actually using `NumPieces` anywhere. Removed it.

https://github.com/llvm/llvm-project/pull/65187