[Sema] -Wpointer-bool-conversion: suppress lambda function pointer conversion...

[Sema] -Wpointer-bool-conversion: suppress lambda function pointer conversion diagnostic during instantiation (#83497)

I have seen two internal pieces of code that uses a template type
parameter to accept any callable type (function pointer, std::function,
closure type, etc). The diagnostic added in #83152 would require
adaptation to the template, which is difficult and also seems
unnecessary. Example:

```cpp
template <typename... Ts>
static bool IsFalse(const Ts&...) { return false; }

template <typename T, typename... Ts,
          typename = typename std::enable_if<std::is_constructible<bool, const T&>::value>::type>
static bool IsFalse(const T& p, const Ts&...) {
  return p ? false : true;
}

template <typename... Args>
void Init(Args&&... args) {
  if (IsFalse(absl::implicit_cast<const typename std::decay<Args>::type&>(
              args)...)) {
    // A callable object convertible to false is either a null pointer or a
    // null functor (e.g., a default-constructed std::function).
    empty_ = true;
  } else {
    empty_ = false;
    new (&factory_) Factory(std::forward<Args>(args)...);
  }
}
```