Fix Qt 5.5 compatibility -- no constFirst/Last methods.

Summary:
QList and QVector both gained these in Qt 5.6, but we claim to support 5.5 (because it's found in Ubuntu 16.04 LTS).

None of these are in performance-relevant code, so I don't believe it's worthwhile adding ifdefs; just revert this patch whenever 5.5 support is dropped.

Reported by 'blaze' on IRC.

Reviewers: KDevelop, mwolff

Subscribers: blaze, kfunk, mwolff, kossebau, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8202