This was only used in a couple of places, and requires extra tracking to
ensure it is correct, while we already have `current_node.lineno`, which
is always accurate and up to date.

I have also fixed a potential strict-null issue by using a sentinel node
for `current_node`

… use it

Which is built, but never used

Since it's only used to check membership, a set makes more sense. Also
add some documentation to the attribute

The former performs a linear search, which is slower than the set based
search.

It's not actually useful, we can just add the root meson.build file to
the depfiles when we translate the meson.build into ast. If there's a
provided ast then we don't go down that path anyway.

The AstInterpreter does this check as well, so don't do it twice.