[TypeLowering] Record pack used in aggregate sig.

Every `LowerType::visit*` function eventually calls through to a
`LowerType::handle*` function.  After
#81581, every
`LowerType::handle*` needs to set the `hasPack` flag based on the
passed-in type by calling `mergeHasPack`.  Add the missing call in the
`handleAggregateByProperties` function.

rdar://152580661

Author: nate-chandler

lib/SIL/IR/TypeLowering.cpp

@@ -2742,6 +2742,7 @@ namespace {
     template <class LoadableLoweringClass>
     TypeLowering *handleAggregateByProperties(CanType type,
                                               RecursiveProperties props) {
+      props = mergeHasPack(HasPack_t(type->hasAnyPack()), props);
       if (props.isAddressOnly()) {
         return handleAddressOnly(type, props);
       }

validation-test/IRGen/rdar152580661.swift

@@ -0,0 +1,14 @@
+// RUN: %target-swift-frontend %s -target %target-swift-5.9-abi-triple -emit-ir
+
+struct H<T> {
+  var packs: [Pack<T>] {
+      id(_packs)
+  }
+  let _packs: [Pack<T>]
+}
+
+struct Pack<each T> {}
+
+func id<T>(_ t: T) -> T {
+  return t
+}