Diagnostic should use Source

[matklad]

In Diagnostic, we have

pub trait Diagnostic {
    fn file(&self) -> HirFileId;
    fn syntax_node_ptr(&self) -> SyntaxNodePtr;
}

I think this should be changed to

pub trait Diagnostic {
    fn source(&self) -> Source<SyntaxNodePtr>;
}

which is the same information, but arranged in a more robust way.

Source is defined here: https://github.com/rust-analyzer/rust-analyzer/blob/e3f8e6023d66ceb37040833bf317d77019bb10f1/crates/ra_hir/src/code_model/src.rs#L9