This should improve performance by not creating unnecessary QString
objects. A QString is never more useful than a QStringView in the
changed code.

Provide forwarding const QString& overloads of IndexedString, Identifier
and QualifiedIdentifier constructors for two reasons:
1. A QStringView is not constructible from a QStringBuilder expression.
   So explicit casting to QString would be necessary:
   -    return Identifier(QLatin1Char('~') + str);
   +    return Identifier(QString{QLatin1Char('~') + str});
2. In kdev-php and possibly in other plugins QString arguments would
   have to be explicitly casted to QStringView in order to avoid
   ambiguous calls of overloaded IndexedString(const QString&) with
   const QUrl& and QStringView candidates. This reason can be eliminated
   by adding -DQT_NO_URL_CAST_FROM_STRING definition to CMakeLists.txt.