Autolink should deal with built-in types differently

[eksperimental]

Let's say I want to link to t:char_list/0 type, ExDoc should remove the "t:".

Currently, since t:char_list/0 cannot be linked to anyhting, then it doesn't do anything to what's in backticks.

This issue came up in: elixir-lang/elixir#5808 (comment)