[clang] [APINotes] Upstream APINotesManager (PR #72389)

[Saleem Abdulrasool via cfe-commits]

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@@ -737,6 +737,9 @@ inline bool operator!=(const TypedefInfo &LHS, const TypedefInfo &RHS) {
   return !(LHS == RHS);
 }
 
+/// The file extension used for the source representation of API notes.
+static const char SOURCE_APINOTES_EXTENSION[] = "apinotes";
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compnerd wrote:

Perhaps add a `constexpr` to this as well?

https://github.com/llvm/llvm-project/pull/72389