Fix Fn supertraits indentation

[Flowrey]

Fixes #6127

In my opinion, the choose_rhs function wrongly presume that the orig_rhs is a single line and so indent only the beginning of the string instead of the whole block.
The changes have made fix the issues for #6127 but l’m really not convinced I've done it the right way 😓

[ytmimi]

@Flowrey thanks so much for taking a look at this one. You're correct this isn't the approach that we'd want to take in order to solve this. We'd want the code that's responsible for rewriting the bound to add the proper indentation.

That said, I remembered that I'd opened #6048 a while ago and after some testing It seems that those code changes also address #6127. I'll be sure to add test cases for #6127 in my PR.