Fix QStringBuilder taking reference to temporary due to auto

Summary:
Compiler decides for "auto" type result to use QStringBuilder, though the
expression passed to it includes a temporary QString result from a
QStringList::join() result. Which then is dropped after the assignement, due
to getting out of scope.

BUG: 394055

Reviewers: #kdevelop, shashwatdixit, mwolff, kfunk

Reviewed By: #kdevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D14235