Backport 3941

[ytmimi]

rewrite generic params no matter what kind it may be (#3941)

[ytmimi]

I want to call out that the following compiles and has empty bound_generic_params

fn foo<T>(_: T) where for<> T: std::fmt::Debug {
    println!("hello");
}


#[derive(Debug)]
struct A; 

fn main() {
    foo(A)
}

Related to #5316, and with the backport changes in this PR, the previous snippet is reformatted to:
(note the removal of for<> in the where clause)

fn foo<T>(_: T)
where
    T: std::fmt::Debug,
{
    println!("hello");
}

#[derive(Debug)]
struct A;

fn main() {
    foo(A)
}

#5316 pertains to bound generics in the context of macro calls, however the removal of the for<> in the where context doesn't produce invalid code. I'm wondering if rustfmt should remove the empty for<> in the where clause context, preserve it in the macro context, or preserve it in both?