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

[Saleem Abdulrasool via cfe-commits]

https://github.com/compnerd approved this pull request.

Given the minor variations, it's likely that this has no real impact outside of simplifying the usage. I suppose that given the existing behavior being different this is fine, though, I would prefer that we had similar behavior across platforms (`__declspec(dllexport)` on windows, `__attribute __((__visibility __("default")))` on other platforms)

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