Adjust rhs shape width to account for next line indentation

[ytmimi]

Fixes #5321

Previously the Shape returned from expr::shape_from_rhs_tactic would always return a Shape with a width == config.max_width when the rhs_tactic was RhsTactics::ForceNextLineWithoutIndent and the indentation was 0. For example, when rewriting generic trait bounds at the top level of a file.

In this case, trait bounds that were between (max_width - tab_spaces) and max_width, would be considered properly formatted and wouldn't wrap.

Now, the shape takes into account the whitespace to properly wrap when the max_width is exceeded.

[calebcartwright]

Need to think on this for a bit to determine whether it meets the criteria of a true bug or if it's at risk of violating our stability guarantee and would need to be gated. It's likely going to end up being the latter even though there's a reasonable case to be made for the former

[ytmimi]

@calebcartwright I went ahead and version gated the changes. If we decide that It doesn't need to be version gated it should be easy to revert the 2nd commit.