[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