Add typepos field to json output of describe command. This field shows where
type of current (under cursor) identifier is defined. This will help code editors
implement command 'Go to type definition'.
Implements [#27308](https://github.com/golang/go/issues/27308)
Change-Id: I4e02ddbdc03fecec98135b8996f9562a88a9cfb8
GitHub-Last-Rev: be47e397a293a96d3d39776d6090d861e7904a24
GitHub-Pull-Request: golang/tools#50
Reviewed-on: https://go-review.googlesource.com/c/140379
Reviewed-by: Alan Donovan <adonovan@google.com>