[clang] [Clang] Fix an integer overflow issue in computing CTAD's parameter depth (PR #128704)

[Haojian Wu via cfe-commits]

hokein wrote:

> (We should have been doing this wrong prior to 20, but it didn't crash because it called Decl::getTemplateDepth(), which returns 0 for the transformed template parameter declaration)

I think you're right. The depth is already incorrect in clang19 etc. https://godbolt.org/z/d9xhxh3EM

```
 |-TemplateTypeParmDecl 0x175b5b88 <line:6:13, col:22> col:22 typename depth 32767 index 1 T
```

https://github.com/llvm/llvm-project/pull/128704