Merge pull request swiftlang#12555 from slavapestov/fix-a-warning

SIL: Fix a warning and add a FIXME

Author: slavapestov

lib/SIL/SILWitnessTable.cpp

@@ -166,10 +166,10 @@ bool SILWitnessTable::conformanceIsSerialized(ProtocolConformance *conformance,
     return true;
 
   auto *nominal = conformance->getType()->getAnyNominal();
-  // Only serialize if the witness table is sufficiently static, and resilience
-  // is explicitly enabled for this compilation or if we serialize all eligible
-  // witness tables.
-  auto moduleIsResilient = strategy == ResilienceStrategy::Resilient;
+  // Only serialize witness tables for fixed layout types.
+  //
+  // FIXME: This is not the right long term solution. We need an explicit
+  // mechanism for declaring conformances as 'fragile'.
   auto protocolIsPublic =
       conformance->getProtocol()->getEffectiveAccess() >= AccessLevel::Public;
   auto typeIsPublic = nominal->getEffectiveAccess() >= AccessLevel::Public;