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

[Tomohiro Kashiwada via cfe-commits]

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@@ -27,6 +27,8 @@
 using namespace clang;
 using namespace ssaf;
 
+LLVM_INSTANTIATE_REGISTRY(llvm::Registry<MockSerializationFormat::FormatInfo>)
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kikairoya wrote:

I'm not sure about the previous discussion about this. Does @compnerd or someone else know?

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