compiletest: Add `LineNumber` newtype to avoid `+1` magic here and there
Start small. If it works well we can increase usage bit by bit as time passes.
My main motivation for doing this is to get rid of the `+ 1` I otherwise have to add in https://github.com/rust-lang/rust/pull/150201 on this line:
```rs
crate::directives::line::line_directive(file, zero_based_line_no + 1, &line)
```
But I think this is a nice general improvement by itself.
Note that we keep using "0" to represent "no specific line" because changing to `Option<LineNumber>` everywhere is a very noisy and significant change. That _can_ be changed later if wanted, but let's not do it now.
c0337ca5f0