[clang] [llvm] [LLVM][Support] Enforce proper instantiations of `Registry<T>` (PR #185141)

[Balázs Benics via cfe-commits]

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@@ -155,23 +176,81 @@ template <typename T, typename... CtorParamTypes> class Registry {
 
 } // end namespace llvm
 
-#ifdef _WIN32
-/// Instantiate a registry class.
-#define LLVM_INSTANTIATE_REGISTRY(REGISTRY_CLASS)                              \
+// Helper macros to instantiate registry class.
+//
+// To provide a `Registry` interface, follow these steps:
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steakhal wrote:

Maybe these comments should be doxygen-style and begin with 3 slashes.

https://github.com/llvm/llvm-project/pull/185141