[clangd][c++20]Consider rewritten binary operators in TargetFinder

[clangd][c++20]Consider rewritten binary operators in TargetFinder

In C++20 some binary operations can be rewritten, e.g. `a != b`
can be rewritten to `!(a == b)` if `!=` is not explicitly defined.
The `TargetFinder` hasn't considered the corresponding `CXXRewrittenBinaryOperator` yet. This resulted that the definition of such operators couldn't be found
when navigating to such a `!=` operator, see https://github.com/clangd/clangd/issues/1476.

In this patch we add support of `CXXRewrittenBinaryOperator` in `FindTarget`.
In such a case we redirect to the inner binary operator of the decomposed form.
E.g. in case that `a != b` has been rewritten to `!(a == b)` we go to the
`==` operator. The `==` operator might be implicitly defined (e.g. by a `<=>`
operator), but this case is already handled, see the new test.

I'm not sure if I the hover test which is added in this patch is the right one,
but at least is passed with this patch and fails without it :)

Note, that it might be a bit missleading that hovering over a `!=` refers to
"instance method operator==".

Differential Revision: https://reviews.llvm.org/D153331