AutoDiff: Check conformances on synthesized TangentVector struct

Once SF.SynthesizedDecls is gone, we can't rely on this happening for us.

Author: slavapestov

lib/Sema/DerivedConformanceDifferentiable.cpp

@@ -490,6 +490,9 @@ getOrSynthesizeTangentVectorStruct(DerivedConformance &derived, Identifier id) {
     member->setImplicit();
 
   derived.addMembersToConformanceContext({structDecl});
+
+  TypeChecker::checkConformancesInContext(structDecl);
+
   return structDecl;
 }