Add __attribute__((format(printf()))) to q(v)nprintf()

They were lacking it since the start of the public history, and
porting to std::snprintf() (which has the warning) as part of making
Qt compile with QT_NO_SNPRINTF has turned up surprisingly many
-Wformat warnings, so enable the warning for the remaining (non-tree)
users.

Backporting aggressively, as this might unearth security bugs in user
code.

[ChangeLog][QtCore][q(v)snprintf] Added attributes for GCC-compatible
compilers to detect format/argument mismatches. If this throws warnings
for your calls now, don't ignore them. printf() format mistakes could
be security-relevant. You may also find that you relied on undocumented
behavior, such as that certain implementations (Windows, Android, WASM)
of qsnprintf() support char16_t* instead of wchar_t* for %ls. In that
case, you should port to qUtf16Printable() and QString::asprintf(), or
suppress the warning and port away from the platform dependence at your
earliest convenience.

Task-number: QTBUG-127110
Pick-to: 6.7 6.5 6.2 5.15
Change-Id: I5c1fd9b2d5d2d55c68773f33edfd76acacd2408c
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Jason McDonald <macadder1@gmail.com>
(cherry picked from commit 64416d3cf645187385d8ad90bc44d9c8e9ce864f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qbytearrayalgorithms.h

@@ -139,8 +139,10 @@ Q_CORE_EXPORT int qstrnicmp(const char *, const char *, size_t len);
 Q_CORE_EXPORT int qstrnicmp(const char *, qsizetype, const char *, qsizetype = -1);
 
 // implemented in qvsnprintf.cpp
-Q_CORE_EXPORT int qvsnprintf(char *str, size_t n, const char *fmt, va_list ap);
-Q_CORE_EXPORT int qsnprintf(char *str, size_t n, const char *fmt, ...);
+Q_CORE_EXPORT int qvsnprintf(char *str, size_t n, const char *fmt, va_list ap)
+    Q_ATTRIBUTE_FORMAT_PRINTF(3, 0);
+Q_CORE_EXPORT int qsnprintf(char *str, size_t n, const char *fmt, ...)
+    Q_ATTRIBUTE_FORMAT_PRINTF(3, 4);
 
 // qChecksum: Internet checksum
 Q_CORE_EXPORT quint16 qChecksum(QByteArrayView data, Qt::ChecksumType standard = Qt::ChecksumIso3309);

tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp

@@ -716,7 +716,11 @@ void tst_QByteArray::qvsnprintf()
 
 #ifndef Q_OS_WIN
     memset(buf, 42, sizeof(buf));
+    QT_WARNING_PUSH
+    QT_WARNING_DISABLE_GCC("-Wformat-zero-length")
+    QT_WARNING_DISABLE_CLANG("-Wformat-zero-length")
     QCOMPARE(::qsnprintf(buf, 10, ""), 0);
+    QT_WARNING_POP
 #endif
 }