IncludePathProperties: prepare to QStringView::sliced()

QStringView::mid() will be ported to QStringView::sliced() once KDevelop
depends on Qt 6. The behavior is undefined if the sum of arguments to
sliced() is greater than the string view's size.

Provide a const QString& overload for the function template
KDevelop::matchesAtOffset(), because optimum performance requires
passing QLatin1String and QStringView by value and QString - by
reference.