[clang] Fix quadratic slowdown in AST matcher parent map generation (PR #87824)

[Aaron Ballman via cfe-commits]

================
@@ -61,7 +61,28 @@ class ParentMapContext::ParentMap {
   template <typename, typename...> friend struct ::MatchParents;
 
   /// Contains parents of a node.
-  using ParentVector = llvm::SmallVector<DynTypedNode, 2>;
+  class ParentVector {
+  public:
+    ParentVector() = default;
+    explicit ParentVector(size_t n, const DynTypedNode &value) {
+      Items.reserve(n);
+      for (; n > 0; --n) {
+        push_back(value);
+      }
+    }
+    bool contains(const DynTypedNode &value) {
+      return Seen.contains(value);
+    }
+    void push_back(const DynTypedNode &value) {
+      if (!value.getMemoizationData() || Seen.insert(value).second) {
+        Items.push_back(value);
+      }
----------------
AaronBallman wrote:

```suggestion
    explicit ParentVector(size_t N, const DynTypedNode &Value) {
      Items.reserve(N);
      for (; N > 0; --N)
        push_back(Value);
    }
    bool contains(const DynTypedNode &Value) {
      return Seen.contains(Value);
    }
    void push_back(const DynTypedNode &Value) {
      if (!Value.getMemoizationData() || Seen.insert(Value).second)
        Items.push_back(Value);
```
Changes to match our coding style, NFC.

Regarding:
```
      for (; N > 0; --N)
        push_back(Value);
```
should we use `std::fill_n` instead?

https://github.com/llvm/llvm-project/pull/87824