[clang] Reduce memory usage in AST parent map generation by lazily checking if nodes have been seen (PR #129934)

[Aaron Ballman via cfe-commits]

================
@@ -70,16 +94,38 @@ class ParentMapContext::ParentMap {
         push_back(Value);
     }
     bool contains(const DynTypedNode &Value) {
-      return Seen.contains(Value);
+      assert(Value.getMemoizationData());
+      bool found = FragileLazySeenCache.contains(&Value);
----------------
AaronBallman wrote:

```suggestion
      bool Found = FragileLazySeenCache.contains(&Value);
```

https://github.com/llvm/llvm-project/pull/129934