Add developer-build with clang 7 (or higher)

When I tried building Qt 5.15 branch with clang 7 and configure switch "-developer-build", I ran into four issues I had to fix. The first one looks especially serious to me, since that constructor must never have worked correctly:

• https://codereview.qt-project.org/c/qt/qtlottie/+/298342
• https://codereview.qt-project.org/c/qt/qtlocation/+/298566
• https://codereview.qt-project.org/c/qt/qtlocation/+/298567
• https://codereview.qt-project.org/c/qt/qtlocation/+/298563

I think it would have been beneficial if the CI system had found these issues immediately.