[clang] [NFC][Clang][OpenMP] Add helper functions/utils for finding/comparing attach base-ptrs. (PR #155625)

[Alexey Bataev via cfe-commits]

================
@@ -6771,6 +6771,240 @@ LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE();
 // code for that information.
 class MappableExprsHandler {
 public:
+  /// Custom comparator for attach-pointer expressions that compares them by
+  /// complexity (i.e. their component-depth) first, then by the order in which
+  /// they were computed by collectAttachPtrExprInfo(), if they are semantically
+  /// different.
+  struct AttachPtrExprComparator {
+    const MappableExprsHandler *Handler;
+    // Cache of previous equality comparison results.
+    mutable llvm::DenseMap<std::pair<const Expr *, const Expr *>, bool>
+        CachedEqualityComparisons;
+
+    AttachPtrExprComparator(const MappableExprsHandler *H) : Handler(H) {}
+
+    // Return true iff LHS is "less than" RHS.
+    bool operator()(const Expr *LHS, const Expr *RHS) const {
+      if (LHS == RHS)
+        return false;
+
+      // First, compare by complexity (depth)
+      auto ItLHS = Handler->AttachPtrComponentDepthMap.find(LHS);
+      auto ItRHS = Handler->AttachPtrComponentDepthMap.find(RHS);
----------------
alexey-bataev wrote:

```suggestion
      const auto ItLHS = Handler->AttachPtrComponentDepthMap.find(LHS);
      const auto ItRHS = Handler->AttachPtrComponentDepthMap.find(RHS);
```

https://github.com/llvm/llvm-project/pull/155625