QStringView: use qstringfwd.h

... instead of fwd-declaring a bunch of stuff manually.

Also move the fake QDoc decalaration of a "class QUtf8StringView"
over.

Pick-to: 6.9 6.8 6.5
Change-Id: I80bc3240d69f69602c127fc0e8fe694dd46765f1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/text/qstringfwd.h

@@ -42,7 +42,9 @@ class QChar;
 class QRegularExpression;
 class QRegularExpressionMatch;
 
-#ifndef Q_QDOC
+#ifdef Q_QDOC
+class QUtf8StringView;
+#else
 // ### Qt 7: remove the non-char8_t version of QUtf8StringView
 QT_BEGIN_NO_CHAR8_T_NAMESPACE
 using QUtf8StringView = QBasicUtf8StringView<false>;

src/corelib/text/qstringview.h

@@ -8,6 +8,7 @@
 #include <QtCore/qcompare.h>
 #include <QtCore/qcontainerfwd.h>
 #include <QtCore/qbytearray.h>
+#include <QtCore/qstringfwd.h>
 #include <QtCore/qstringliteral.h>
 #include <QtCore/qstringalgorithms.h>
 
@@ -22,13 +23,8 @@ Q_FORWARD_DECLARE_OBJC_CLASS(NSString);
 
 QT_BEGIN_NAMESPACE
 
-class QString;
-class QStringView;
 class QRegularExpression;
 class QRegularExpressionMatch;
-#ifdef Q_QDOC
-class QUtf8StringView;
-#endif
 
 namespace QtPrivate {
 template <typename Char>