Get rid of the second call to setType() in validateExtension().

This means that we no longer have the invariant that the extendedType always
contains the generic parameters. So we need to fix the assertions/test cases
for it.

lib/Serialization/Serialization.cpp

@@ -2871,7 +2871,6 @@ public:
 
     auto contextID = S.addDeclContextRef(extension->getDeclContext());
     Type baseTy = extension->getExtendedType();
-    assert(!baseTy->hasUnboundGenericType());
     assert(!baseTy->hasArchetype());
 
     // FIXME: Use the canonical type here in order to minimize circularity