[clang-format] Propagate `LeadingEmptyLinesAffected` when joining lines (#146761)
Before this commit, when `LineJoiner` joins a line with affected leading
whitespace, it would drop the knowledge of this entirely. However, when
the `AffectedRangeManager` is computing the affected lines, the leading
empty whitespace lines are potentially considered for non-first tokens
in the `AnnotatedLine`. This causes a discrepancy in behavior when an
`AnnotatedLine` is put together from joining multiple lines versus when
it is not.
We change `LineJoiner::join` to follow `AffectedRangeManager`'s logic,
considering the leading whitespace when determining `Affected` for a
token.
https://github.com/llvm/llvm-project/blob/a63f57262898588b576d66e5fd79c0aa64b35f2d/clang/lib/Format/AffectedRangeManager.cpp#L111-L130
Fixes #138942.