Fix a crash when serializing variadic generic tuple code under -wmo

It seems really unfortunate that we use SILCloner to, basically,
implement a recursive visitor of the types used in a SIL function,
but apparently it's what we do.

Fixes apple#72117.

include/swift/SIL/SILCloner.h

@@ -293,14 +293,18 @@ class SILCloner : protected SILInstructionVisitor<ImplClass> {
  continue;
  }
 
- // Substitute the shape class of the expansion.
+ // Substitute the shape class of the expansion. If this doesn't
+ // give us a pack (e.g. if this isn't a substituting clone),
+ // we're never erasing tuple structure.
  auto newShapeClass = getOpASTType(expansion.getCountType());
  auto newShapePack = dyn_cast<PackType>(newShapeClass);
+ if (!newShapePack)
+ return false;
 
  // If the element has a name, then the tuple sticks around unless
  // the expansion disappears completely.
  if (type->getElement(index).hasName()) {
- if (newShapePack && newShapePack->getNumElements() == 0)
+ if (newShapePack->getNumElements() == 0)
  continue;
  return false;
  }

test/SILOptimizer/Inputs/cross-module/cross-module.swift

@@ -295,3 +295,13 @@ public func getEmptySet() -> Set<Int> {
  return Set()
 }
 
+public protocol Visitable {
+ func visit()
+}
+@available(SwiftStdlib 6.0, *)
+public struct S<each T : Visitable> {
+ var storage: (repeat each T)
+ public func visit() {
+ _ = (repeat (each storage).visit())
+ }
+}