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

[Michael Buch via cfe-commits]

Michael137 wrote:

> It is a realistic requirement that new similar parameters are added, I had already an (experimental) fix where this is needed. Still I like better the solution with separate caches because efficiency reasons (`IgnoreTemplateParmDepth = true` is rarely used, not at all for C code). For new parameters we have the similar set of choices (add more new caches or extend cache key or not use the cache at all).

We wouldn't just have to add a single extra cache, but each parameter would require us to add caches for all permutations of the parameters. Also, I'd be surprised if there's a noticeable difference in performance between the two to be honest. So I still prefer your first solution, but I don't want to block this if you prefer the other approach

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