[clang] [clang][Index] Add comment about out-of-line defaulted destructor (PR #138640)

[Shafik Yaghmour via cfe-commits]

================
@@ -44,6 +44,8 @@ class IndexingContext {
 
 public:
   IndexingContext(IndexingOptions IndexOpts, IndexDataConsumer &DataConsumer);
+  // Defaulted, but defined out of line to avoid a dependency on
+  // HeuristicResolver.h.
----------------
shafik wrote:

```suggestion
  // HeuristicResolver.h. Because unique_ptr requires a complete
  // type at the point where it's destructor would be called.
```

https://github.com/llvm/llvm-project/pull/138640