This is a quick way to implement unresolved reference diagnostics.
For example, adding to VS Code config
"editor.tokenColorCustomizationsExperimental": {
"unresolvedReference": "#FF0000"
},
will highlight all unresolved refs in red.
This introduces the new type -- Semantics.
Semantics maps SyntaxNodes to various semantic info, such as type,
name resolution or macro expansions.
To do so, Semantics maintains a HashMap which maps every node it saw
to the file from which the node originated. This is enough to get all
the necessary hir bits just from syntax.
