support for $this as an array when implementing ArrayAccess

Summary:
Accessing $this as an array if interface ArrayAccess is implemented
[[ https://bugs.kde.org/show_bug.cgi?id=373414 | bug373414 ]] / [[ https://bugs.kde.org/show_bug.cgi?id=346876 | bug346876 ]]

Reviewers: #kdevelop, mwolff, zhigalin, kfunk

Reviewed By: #kdevelop, zhigalin, kfunk

Subscribers: kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D4776

(cherry picked from commit 1f15b3b3)