Do not unindent code block in comments with unformattable macro

[topecongiro]

format_code_block formats the given code_snippet by enclosing it inside
fn main block. Previously we did not add indentation to the code_snippet
before formatting it. This works fine as long as we can format the given
code_snippet, but when the code block has unformattable macro, they gets
unindented. This PR fixes it by adding proper indentation before formatting
the code_snippet.

For example, when formatting the following code block,

some_macro!(pub fn foo() {
    println!("Don't unindent me!");
});

previously we enclosed it like this:

fn main() {
some_macro!(pub fn foo() {
    println!("Don't unindent me!");
});
}

with this PR, we will enclose it like this:

fn main() {
    some_macro!(pub fn foo() {
        println!("Don't unindent me!");
    });
}

Closes #2523.

[nrc]

Thank you!