The scope map models all of the name lookup scopes within a source
file. It can be queried by source location to find the innermost scope
that contains that source location. Then, one can follow the parent
pointers in the scope to enumerate the enclosing scopes.
The scope map itself is lazily constructed, only creating scope map
nodes when required implicitly (e.g, when searching for a particular
innermost scope) or forced for debugging purposes.
using a lazily-constructed tree that can be searched by source
location. A search within a particular source location will
