[clang][Sema] Populate function template depth at AddTemplateOverloadCandidate (#80395)

This is yet another one-line patch to fix crashes on constraint
substitution.

```cpp
template <class, class> struct formatter;

template <class, class> struct basic_format_context {};

template <typename CharType>
concept has_format_function = format(basic_format_context<CharType, CharType>());

template <typename ValueType, typename CharType>
  requires has_format_function<CharType>
struct formatter<ValueType, CharType> {
  template <typename OutputIt>
  CharType format(basic_format_context<OutputIt, CharType>);
};
```

In this case, we would build up a `RecoveryExpr` for a call within a
constraint expression due to the absence of viable functions. The
heuristic algorithm attempted to find such a function inside of a
ClassTemplatePartialSpecialization, from which we started to substitute
its requires-expression, and it succeeded with a FunctionTemplate such
that

1) It has only one parameter...