Distinguish "declared" and "inferred" `Place`s

[sharkdp]

A Place is currently either Place::Unbound, Place::Type(type, Boundness::PossiblyUnbound or Place::Type(type, Boundness::Bound):

pub(crate) enum Boundness {
    Bound,
    PossiblyUnbound,
}

pub(crate) enum Place<'db> {
    Type(Type<'db>, Boundness),
    Unbound,
}

There is currently no way of tracking if a Place originated from a declaration or if its type has been inferred. However, we still use e.g. Place::Type(type, Boundness::Bound) to mean "definitely declared" in various places. For example, the place_from_declarations query also returns a Place (wrapped in PlaceFromDeclarationsResult -> PlaceAndQualifiers -> Place).

Similarly, the Member return type of various member-access functions currently tracks the "declared vs inferred" information separately.

We should attempt to refactor Place to generally contain metadata about the origin of a particular type.

Related issue: #1051

[mtshiba]

I remember considering the related issue during astral-sh/ruff#18041. At the time, it was associated with #229.
The new API for Place I was considering at the time was as follows:

// Boundness renamed
pub(crate) enum Definedness {
    Defined, // Bound renamed
    PossiblyUndefined, // PossiblyUnbound renamed
}

pub(crate) enum Place<'db> {
    // If there is both a declaration and a binding, `Place::Bound` will be used.
    Bound(Type<'db>, Definedness),
    Declared(Type<'db>, Definedness),
    Undefined,
}

If no one has started working on this yet, I'll get to it.
Please let me know if you have any comments on the direction of the new API.