Move the rest of macros from qglobal to QtPreprocessorSupport

We had only three macros left in QtGlobal:
* QT_STRINGIFY
* Q_UNUSED()
* Q_UNIMPLEMENTED()

There is no obvious existing header for them, so a new header is
created.
As a drive-by: add documentation for QT_STRINGIFY.

Task-number: QTBUG-106154
Task-number: QTBUG-99313
Change-Id: I2d051dd3e69f13602893a0f0d6968419479b6c23
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/CMakeLists.txt

@@ -81,6 +81,7 @@ qt_internal_add_module(Core
         global/qtconfigmacros.h
         global/qtdeprecationmarkers.h
         global/qtenvironmentvariables.cpp global/qtenvironmentvariables.h
+        global/qtpreprocessorsupport.h
         global/qtrace_p.h
         global/qtresource.h
         global/qttranslation.h

src/corelib/global/qglobal.cpp

@@ -227,16 +227,6 @@ void qAbort()
 // localtime() -- but not localtime_r(), which we use when threaded
 // strftime() -- not used (except in tests)
 
-/*!
-    \macro Q_UNUSED(name)
-    \relates <QtGlobal>
-
-    Indicates to the compiler that the parameter with the specified
-    \a name is not used in the body of a function. This can be used to
-    suppress compiler warnings while allowing functions to be defined
-    with meaningful parameter names in their signatures.
-*/
-
 struct QInternal_CallBackTable
 {
     QList<QList<qInternalCallback>> callbacks;

src/corelib/global/qglobal.h

@@ -25,10 +25,7 @@
 #include <QtCore/qtconfigmacros.h>
 #include <QtCore/qtcoreexports.h>
 
-/* These two macros makes it possible to turn the builtin line expander into a
- * string literal. */
-#define QT_STRINGIFY2(x) #x
-#define QT_STRINGIFY(x) QT_STRINGIFY2(x)
+#include <QtCore/qtpreprocessorsupport.h>
 
 inline void qt_noop(void) {}
 
@@ -40,22 +37,10 @@ inline void qt_noop(void) {}
 #include <QtCore/qtypes.h>
 #include <QtCore/qtclasshelpermacros.h>
 
-/*
-   Avoid "unused parameter" warnings
-*/
-#define Q_UNUSED(x) (void)x;
 
 #ifndef __ASSEMBLER__
-QT_BEGIN_NAMESPACE
-
 #if defined(__cplusplus)
 
-#if !defined(Q_UNIMPLEMENTED)
-#  define Q_UNIMPLEMENTED() qWarning("Unimplemented code.")
-#endif
-
-QT_END_NAMESPACE
-
 // We need to keep QTypeInfo, QSysInfo, QFlags, qDebug & family in qglobal.h for compatibility with Qt 4.
 // Be careful when changing the order of these files.
 #include <QtCore/qtypeinfo.h>

src/corelib/global/qtpreprocessorsupport.h

@@ -0,0 +1,26 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+
+#ifndef QTPREPROCESSORSUPPORT_H
+#define QTPREPROCESSORSUPPORT_H
+
+#if 0
+#pragma qt_class(QtPreprocessorSupport)
+#pragma qt_sync_stop_processing
+#endif
+
+/* These two macros makes it possible to turn the builtin line expander into a
+ * string literal. */
+#define QT_STRINGIFY2(x) #x
+#define QT_STRINGIFY(x) QT_STRINGIFY2(x)
+
+/*
+   Avoid "unused parameter" warnings
+*/
+#define Q_UNUSED(x) (void)x;
+
+#if !defined(Q_UNIMPLEMENTED)
+#  define Q_UNIMPLEMENTED() qWarning("Unimplemented code.")
+#endif
+
+#endif // QTPREPROCESSORSUPPORT_H

src/corelib/global/qtpreprocessorsupport.qdoc

@@ -0,0 +1,20 @@
+// Copyright (C) 2020 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
+
+/*!
+    \macro Q_UNUSED(name)
+    \relates <QtPreprocessorSupport>
+
+    Indicates to the compiler that the parameter with the specified
+    \a name is not used in the body of a function. This can be used to
+    suppress compiler warnings while allowing functions to be defined
+    with meaningful parameter names in their signatures.
+*/
+
+/*!
+    \macro QT_STRINGIFY(arg)
+    \relates <QtPreprocessorSupport>
+
+    The macro can be used to turn the builtin line expander \a arg into a string
+    literal.
+*/