SIL: Fix substWitnessConformance() to set SubstFlags::PreservePackExpansionLevel

Author: slavapestov

lib/SIL/IR/SILTypeSubstitution.cpp

@@ -273,9 +273,10 @@ class SILTypeSubstituter :
     // Substitute the underlying conformance of opaque type archetypes if we
     // should look through opaque archetypes.
     if (typeExpansionContext.shouldLookThroughOpaqueTypeArchetypes()) {
-      auto substType = IFS.withNewOptions(std::nullopt, [&] {
-        return selfType.subst(IFS)->getCanonicalType();
-      });
+      auto substType = IFS.withNewOptions(
+        SubstFlags::PreservePackExpansionLevel, [&] {
+          return selfType.subst(IFS)->getCanonicalType();
+        });
       if (substType->hasOpaqueArchetype()) {
         substConformance = substOpaqueTypesWithUnderlyingTypes(
             substConformance, typeExpansionContext);