Include function return types in scope tree

This PR adds a couple more bits:

1. The return type is included in the scope tree,
2. The trailing return type (if declared as e.g. `-> decltype()`) is returned as well,
3. Fill in a missing bit so that `lambda`s properly get their icon.

I think this is probably the limit as far as pure-JavaScript solutions go here; if I revisit this later I'll investigate making async calls to libclang so we can get some nicer formatting, handling of macros, and so on.