[clang-format] Fix bug in parsing `operator<` with template

Fixes https://github.com/llvm/llvm-project/issues/44601.

This patch handles a bug when parsing a below example code :

```
template <class> class S;

template <class T> bool operator<(S<T> const &x, S<T> const &y) {
  return x.i < y.i;
}

template <class T> class S {
  int i = 42;
  friend bool operator< <>(S const &, S const &);
};

int main() { return S<int>{} < S<int>{}; }
```
which parse `< <>` as `<< >`, not `< <>` in terms of tokens as discussed in discord.

1. Add a condition in `tryMergeLessLess()` considering `operator` keyword and `>`
2. Force to leave a whitespace between `tok::less` and a template opener
3. Add unit test

Reviewed By: MyDeveloperDay, curdeius

Differential Revision: https://reviews.llvm.org/D117398