Assignment LHS to RHS Comments handling when rewrite_assign_rhs is used

[davidBar-On]

The places where rewrite_assign_rhsh()/_with() are used, and the comments between lhs and rhs are not handled, were identified in #4626. I submitted PRs to use rewrite_assign_rhs_with_comments() for the rewrite_assignment and format_trait cases.

For the following cases, it is not clear if and what should be done. There seem to be three options:

1. Do nothing, as it is not expected that comments will be added between the lhs (... =) and rhs in these cases.
2. Enhance the code to use rewrite_assign_rhs_with_comments instead of rewrite_assign_rhs....
3. Only check whether all comments are included in the formatted code and if not use the original code (using recover_comment_removed()).

Following are the relevant cases with code examples when I was able to find an example:

1. format_variant()

enum Foo {
    Bar,
    Baz = /* Block comment */ 123,
    Quux = // Line comment
	124,
}

2. format_trait_alias()

#![feature(trait_alias)]
trait Foo =/*comment*/std::fmt::Debug + Send;
trait Bar =/*comment*/Foo + Sync;

3. rewrite_type()

type Kilometers =/*comment*/i32;

4. rewrite_struct_field()
   Could not find an example. Per this code requires new line inside of field type.

5. rewrite_opaque_impl_type()
   Could not find an example.

6. Static ast::ForeignItem
   Could not find an example.

7. Macro format_lazy_static()
   Maybe this case is not relevant currently, as format_lazy_static() is not called if the lazy_static! macro contains a comment - see here.

8. ast::WherePredicate::BoundPredicate
   Could not find an example.

9. ast::WherePredicate::EqPredicate
   Could not find an example.