struct ctor ids

[Xanewok]

Originally created by @nrc (rust-dev-tools/rls-analysis#138):

are missing from the JSON save-analysis data, but unit and tuple structs will have a reference to the ctor id, not the struct id, which means that we miss a cross-reference.

[Xanewok]

@nrc not sure I understand the problem:

struct Struct { inner: () }
enum MyEnum {
    VariantA,
    VariantB(u32)
}

fn main() {
    let _ = Struct { inner: () };
    let a = MyEnum::VariantA;
    let a = MyEnum::VariantB;
    let a = MyEnum::VariantB(32);
}

Rename and goto-def works both ways on Struct and MyEnum, as well as inner, VariantA and VariantB