[lit] Fix shell commands with newlines (#67898)

In PR #65242 (landed as 9e739fdb), I
claimed that RUN lines cannot contain newlines. Actually, they can after
substitution expansion. More generally, a lit config file can define
substitutions or preamble commands containing newlines. While both of
those cases seem unlikely in practice,
[D154987](https://reviews.llvm.org/D154987) proposes PYTHON directives
where it seems very likely.

Regardless of the use case, without this patch, such newlines break
expansion of `%dbg(RUN: at line N)`, and the fix is simple.