[clang] [clang][AST] Add 'IgnoreTemplateParmDepth' to structural equivalence cache (PR #115518)

[Donát Nagy via cfe-commits]

================
@@ -39,6 +39,10 @@ enum class StructuralEquivalenceKind {
 };
 
 struct StructuralEquivalenceContext {
+  /// Store declaration pairs already found to be non-equivalent.
+  /// key: (from, to, IgnoreTemplateParmDepth)
+  using NonEquivalentDeclSet = llvm::DenseSet<std::tuple<Decl *, Decl *, int>>;
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NagyDonat wrote:

```suggestion
    using NonEquivalentDeclSet =
        std::array<llvm::DenseSet<std::pair<Decl *, Decl *>>, 2>;
```
As above.

https://github.com/llvm/llvm-project/pull/115518