[clang] [Clang] Initialize AtLeastAsSpecialized to prevent undefined behavior in Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs() (PR #95195)

[via cfe-commits]

cor3ntin wrote:

as @mizvekov said, there is no UB here.
A better improvement would be to let `runWithSufficientStackSpace` return a value, so we can write

```cpp
bool AtLeastAsSpecialized =  runWithSufficientStackSpace(/*...*/);
```

https://github.com/llvm/llvm-project/pull/95195