[3.7] bpo-39031: Include elif keyword when producing lineno/col-offset info for if_st...

[3.7] bpo-39031: Include elif keyword when producing lineno/col-offset info for if_stmt (GH-17582) (#17584)

When parsing an "elif" node, lineno and col_offset of the node now point to the "elif" keyword and not to its condition, making it consistent with the "if" node.

https://bugs.python.org/issue39031

Automerge-Triggered-By: @pablogsal.
(cherry picked from commit 025a602af7ee284d8db6955c26016f3f27d35536)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>