[cxx-interop] Fix unavailable generics triggering compilation error

In some cases, the reverse interop generated both a forward declaration and a
definition with unavailable attribute in the C++ header. Unfortunately, the
kinds of these symbol did not match. The forward declaration was templated
while the definition was not. The forward declaration has the correct kind,
so this patch extends the printing of unavailable definitions to include the
generic arguments.

rdar://119835933

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -874,6 +874,11 @@ public:
       // Emit an unavailable stub for a Swift type.
       if (auto *nmtd = dyn_cast<NominalTypeDecl>(vd)) {
         auto representation = cxx_translation::getDeclRepresentation(vd);
+        if (nmtd->isGeneric()) {
+          auto genericSignature =
+              nmtd->getGenericSignature().getCanonicalSignature();
+          ClangSyntaxPrinter(os).printGenericSignature(genericSignature);
+        }
         os << "class ";
         ClangSyntaxPrinter(os).printBaseName(vd);
         os << " { } SWIFT_UNAVAILABLE_MSG(\"";