AST: The conforming type of a normal conformance should be the self, not declared, interface type

Builtin.TheTupleType's self interface type is (T...), and the declared
interface type is the non-substitutable Builtin.TheTupleType.

Author: slavapestov

lib/AST/ConformanceLookupTable.cpp

@@ -875,7 +875,7 @@ ConformanceLookupTable::getConformance(NominalTypeDecl *nominal,
         ctx.getInheritedConformance(type, inheritedConformance.getConcrete());
   } else {
     // Create or find the normal conformance.
-    Type conformingType = conformingDC->getDeclaredInterfaceType();
+    Type conformingType = conformingDC->getSelfInterfaceType();
     SourceLoc conformanceLoc
       = conformingNominal == conformingDC
           ? conformingNominal->getLoc()