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

[Donát Nagy via cfe-commits]

================
@@ -62,7 +62,8 @@ class TypeSourceInfo;
   class ASTImporter {
     friend class ASTNodeImporter;
   public:
-    using NonEquivalentDeclSet = llvm::DenseSet<std::pair<Decl *, Decl *>>;
+    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>;
```
If you use two `DenseSet`s and put them into an array, you can use them without significant code changes.

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