QString: overload the += operator to handle QUtf8StringView

The += operator is already overloaded to handle QStringView and
QLatin1String - add the missing QUtf8StringView overload.

[ChangeLog][QtCore][QString] Added operator+=(QUtf8StringView)
overload.

Task-number: QTBUG-103302
Change-Id: Iec6940bad7866310c826a130b98accebc3c82aa8
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/text/qstring.cpp

@@ -6195,7 +6195,14 @@ QString& QString::fill(QChar ch, qsizetype size)
 
     \overload operator+=()
 
-    Appends the Latin-1 string \a str to this string.
+    Appends the Latin-1 string view \a str to this string.
+*/
+
+/*! \fn QString &QString::operator+=(QUtf8StringView str)
+    \since 6.5
+    \overload operator+=()
+
+    Appends the UTF-8 string view \a str to this string.
 */
 
 /*! \fn QString &QString::operator+=(const QByteArray &ba)

src/corelib/text/qstring.h

@@ -706,6 +706,7 @@ public:
     inline QString &operator+=(const QString &s) { return append(s); }
     inline QString &operator+=(QStringView v) { return append(v); }
     inline QString &operator+=(QLatin1StringView s) { return append(s); }
+    QString &operator+=(QUtf8StringView s) { return append(s); }
 
     QString &remove(qsizetype i, qsizetype len);
     QString &remove(QChar c, Qt::CaseSensitivity cs = Qt::CaseSensitive);

tests/auto/corelib/text/qstring/tst_qstring.cpp

@@ -440,6 +440,8 @@ private slots:
     void operator_pluseq_qstringview_data()   { operator_pluseq_data(EmptyIsNoop); }
     void operator_pluseq_qlatin1string()      { operator_pluseq_impl<QLatin1String, QString &(QString::*)(QLatin1String)>(); }
     void operator_pluseq_qlatin1string_data() { operator_pluseq_data(Latin1Encoded); }
+    void operator_pluseq_qutf8stringview()    { operator_pluseq_impl<QUtf8StringView, QString &(QString::*)(QUtf8StringView)>(); }
+    void operator_pluseq_qutf8stringview_data() { operator_pluseq_data(); }
     void operator_pluseq_qchar()              { operator_pluseq_impl<QChar, QString &(QString::*)(QChar)>(); }
     void operator_pluseq_qchar_data()         { operator_pluseq_data(EmptyIsNoop); }
     void operator_pluseq_qbytearray()         { operator_pluseq_impl<QByteArray>(); }