[clang] [clang][ssaf] Fix building on gcc-7 and MinGW/Cygwin (PR #181812)

[Tomohiro Kashiwada via cfe-commits]

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@@ -42,4 +44,9 @@ class MockSerializationFormat final : public SerializationFormat {
 
 } // namespace clang::ssaf
 
+namespace llvm {
+extern template class CLANG_TEMPLATE_ABI
+    Registry<clang::ssaf::MockSerializationFormat::FormatInfo>;
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kikairoya wrote:

With reverting that, I guess that the linker reports `Registry<...>::Head` is undefined if the instantiation were absent.

https://github.com/llvm/llvm-project/pull/181812