compiletest: Add LineNo newtype to avoid +1 magic here and there

[Enselic]

Start small and only use it in debugger.rs for now. If it works well we use it in more and more places over time.

This takes us one step closer to getting rid of the + 1 I add in #150201 on this line:

                crate::directives::line::line_directive(file, zero_based_line_no + 1, &line)

if we also start using LineNo in DirectiveLine.

Edit: Hmm, in fact, let me try that before asking for review on this PR. Setting to Draft.

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

[rustbot]

r? @jieyouxu

rustbot has assigned @jieyouxu.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer