AST: Pass allowMissing=true when filling in signature conformances

This fixes an AST verifier failure where code that we otherwise
accept (because of loose Sendable checking) causes an assertion
because an invalid conformance shows up in the signature
conformances list of a valid (from the type checker's POV)
normal conformance.

Author: slavapestov

lib/AST/ProtocolConformance.cpp

@@ -873,7 +873,8 @@ void NormalProtocolConformance::finishSignatureConformances() {
     if (substTy->hasTypeParameter())
       substTy = getDeclContext()->mapTypeIntoContext(substTy);
 
-    reqConformances.push_back(module->lookupConformance(substTy, reqProto)
+    reqConformances.push_back(module->lookupConformance(substTy, reqProto,
+                                                        /*allowMissing=*/true)
                                   .mapConformanceOutOfContext());
   }
   setSignatureConformances(reqConformances);