trailing comma after function call chains may need a comment barrier

When given a function call chain, and a comment rustfmt will move the comma above the comment,
which cause

fn main() {
    ().foo(
        ().foo
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            .foo()
            //.foo()
            ,
    )
}

afterwords the last line looks like:
.foo(), //.foo()

Which moves the comment from the first parameter, to the second (nonexistent!) parameter if one were to uncomment it,
This makes the addition of a comment, followed by rustfmt, followed by the removal of a comment syntactically invalid which can be inconvenient.

Start by reproducing the formatting change with the function-call chain and commented-out .foo() shown in the issue. Trace the formatter entry point that handles trailing commas and comments, then add coverage for this example; done means formatting preserves a comment barrier so uncommenting the call remains syntactically valid.