swift234

[Scope map] Introduce an operation to find the innermost scope based on a given source location.

Given a source location, we can find the innermost enclosing scope
that describes that source location. Introduce this operation into the
scope map, then add a testing mode where we probe the scope map at
specifi locations to see what we find. Test for:

1) Finding the right innermost enclosing scope, and
2) That we're only expanding the part of the scope map that is needed
to identify that scope.