AST: Fix substOpaqueTypesWithUnderlyingTypes() to set SubstFlags::PreservePackExpansionLevel

The "iterate until fixed point" would cause an infinite loop where we
wrap type parameter packs in a PackElementType and increase the level
forever otherwise.

This was a regression from commit 103428f.

Fixes rdar://160804717.

Author: slavapestov

lib/AST/TypeSubstitution.cpp

@@ -1093,7 +1093,8 @@ ProtocolConformanceRef swift::substOpaqueTypesWithUnderlyingTypes(
       context.getContext(), context.getResilienceExpansion(),
       context.isWholeModuleContext());
   InFlightSubstitution IFS(replacer, replacer,
-                           SubstFlags::SubstituteOpaqueArchetypes);
+                           SubstFlags::SubstituteOpaqueArchetypes |
+                           SubstFlags::PreservePackExpansionLevel);
 
   auto substRef = ref.subst(IFS);
 

test/Generics/rdar160804717.swift

@@ -0,0 +1,27 @@
+// RUN: %target-swift-frontend -emit-ir %s -target %target-swift-5.9-abi-triple
+// REQUIRES: objc_interop
+
+// This used to trigger an infinite loop in conformance substitution
+// when emitting the opaque type descriptor in IRGen.
+
+// rdar://160804717
+
+import SwiftUI
+
+public struct CategorySplitView<each Destination: View>: View {
+    let titleKey: LocalizedStringKey
+    let groups: (repeat each Destination)
+
+    public var body: some View {
+        TupleView((repeat CategoryGroupSidebar(group: each groups))).navigationTitle(titleKey)
+    }
+}
+
+public struct CategoryGroupSidebar<Destination>: View {
+    let group: Destination
+
+    public var body: some View {
+        Text("Hi")
+    }
+}
+