QString: mark qustrchr() and qustrlen() Q_NEVER_INLINE

Those are "public-ish" and exported functions (qstringalgorithms.h), so
one wouldn't expect that they get inlined most of the time. But they can
be inlined by other code inside qstring.cpp and if QtCore is compiled
with LTO.

More importantly, we want the compiler to inline qustrXXX_avx2() into
it, instead of tail-calling into qustrXXX_avx2(). This may seem like a
no-op but has one very important difference: backtraces will point to an
exported, function instead of an internal symbol. This will then allow
us to create valgrind suppressions.

Before unstripped:
==12492== Invalid read of size 32
==12492==    at 0x53142EE: qustrchr_avx2(QStringView, char16_t) [clone .lto_priv.0] (in /home/tjmaciei/obj/qt/installed/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==12492==    by 0x54A7AE6: QString::indexOf(QChar, long long, Qt::CaseSensitivity) const [clone .constprop.0] (in /home/tjmaciei/obj/qt/installed/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==12492==    by 0x5218EC4: treatAsAbsolute(QString const&) (in /home/tjmaciei/obj/qt/installed/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==12492==    by 0x5218FBA: QDir::filePath(QString const&) const (in /home/tjmaciei/obj/qt/installed/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)

Before, stripped:
==19477== Invalid read of size 32
==19477==    at 0x5314395: ??? (in /home/tjmaciei/obj/qt/installed/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==19477==    by 0x54A7AE6: ??? (in /home/tjmaciei/obj/qt/installed/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==19477==    by 0x5218EC4: ??? (in /home/tjmaciei/obj/qt/installed/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==19477==    by 0x5218FBA: QDir::filePath(QString const&) const (in /home/tjmaciei/obj/qt/installed/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)

After:
==19692== Invalid read of size 32
==19692==    at 0x53143C5: QtPrivate::qustrchr(QStringView, char16_t) (in /home/tjmaciei/obj/qt/qt6-release/qtbase/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==19692==    by 0x54ABA46: ??? (in /home/tjmaciei/obj/qt/qt6-release/qtbase/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==19692==    by 0x5218824: ??? (in /home/tjmaciei/obj/qt/qt6-release/qtbase/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==19692==    by 0x521891A: QDir::filePath(QString const&) const (in /home/tjmaciei/obj/qt/qt6-release/qtbase/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==19692==  Address 0x741919e is 2 bytes before a block of size 32 alloc'd
==19692==    at 0x4843794: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==19692==    by 0x54A8A84: ??? (in /home/tjmaciei/obj/qt/qt6-release/qtbase/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)
==19692==    by 0x531EB1C: QString::fromLatin1(QByteArrayView) (in /home/tjmaciei/obj/qt/qt6-release/qtbase/lib64/glibc-hwcaps/x86-64-v3/libQt6Core.t.so.6.7.0)

Recommended suppressions:
{
   qustrchr_avx2
   Memcheck:Addr32
   fun:_ZN9QtPrivate8qustrchrE11QStringViewDs
}
{
   qustrchr_avx2-with-debuginfo
   Memcheck:Addr32
   fun:UnknownInlinedFun
   fun:UnknownInlinedFun
   fun:_ZN9QtPrivate8qustrchrE11QStringViewDs
}

Change-Id: Ib84fc618ac204205bf7afffd1772b1d87343dfd0
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/corelib/text/qstring.cpp

@@ -683,6 +683,7 @@ static int ucstrncmp_sse2(const char16_t *a, const Char *b, size_t l)
 }
 #endif
 
+Q_NEVER_INLINE
 qsizetype QtPrivate::qustrlen(const char16_t *str) noexcept
 {
 #if defined(__SSE2__) && !(defined(__SANITIZE_ADDRESS__) || __has_feature(address_sanitizer))
@@ -711,6 +712,7 @@ qsizetype QtPrivate::qustrnlen(const char16_t *str, qsizetype maxlen) noexcept
  * character is not found, this function returns a pointer to the end of the
  * string -- that is, \c{str.end()}.
  */
+Q_NEVER_INLINE
 const char16_t *QtPrivate::qustrchr(QStringView str, char16_t c) noexcept
 {
     const char16_t *n = str.utf16();