Review our use of Qt string classes

[oktal3700]

Qt has added a class QStringView, the use of which as a function parameter can avoid constructing a QString in many cases. Since one of swift's main performance bottlenecks is QString constructions, it would be nice to review which of our functions can be refactored to take a QStringView parameter.

It might make sense to also consider where we can make more use of QStringLiteral, QLatin1String, QStringRef, and QStringBuilder.