q{,Utf8}Printable: avoid creating a copy of a QString

We have this QString() constructor call to permit things that convert to
QString but aren't QString to be used in qPrintable, like a
QStringBuilder-powered fast operator+ expression, like:

    qPrintable(string1 + ": " + string2)

Unfortunately, it meant that we unnecessarily created a QString copy if
the input was already QString.

Change-Id: Iecab8770aa5840aba8edfffd1516bc94cec791a9
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal.h

@@ -747,12 +747,13 @@ Q_CORE_EXPORT Q_DECL_CONST_FUNCTION bool qSharedBuild() Q_DECL_NOTHROW;
 #  define QT_DEBUG
 #endif
 
+// QtPrivate::asString defined in qstring.h
 #ifndef qPrintable
-#  define qPrintable(string) QString(string).toLocal8Bit().constData()
+#  define qPrintable(string) QtPrivate::asString(string).toLocal8Bit().constData()
 #endif
 
 #ifndef qUtf8Printable
-#  define qUtf8Printable(string) QString(string).toUtf8().constData()
+#  define qUtf8Printable(string) QtPrivate::asString(string).toUtf8().constData()
 #endif
 
 /*

src/corelib/tools/qstring.h

@@ -1857,6 +1857,12 @@ QT_DEPRECATED inline QString escape(const QString &plain) {
 #endif
 }
 
+namespace QtPrivate {
+// used by qPrintable() and qUtf8Printable() macros
+inline const QString &asString(const QString &s)    { return s; }
+inline QString &&asString(QString &&s)              { return std::move(s); }
+}
+
 QT_END_NAMESPACE
 
 #if defined(QT_USE_FAST_OPERATOR_PLUS) || defined(QT_USE_QSTRINGBUILDER)