Each annotation came from a single top most file. Walk up the loc chain.

STL contains a lot of forwarding headers. For example map forwards to bits/stl_map.
However we still need to annotate the decls that came from stl_map as if they
came from map, because semantically we would like to autoload/show hints about
map and not some 'interesting' stl implementation.

This is why we need to reconstruct the include chain and write out the second
includer. The first one is the prompt's input_line_N.