QDirListing: Extract Method next() from const_iterator::op++()

Having the actual const_iterator::operator++() as an out-of-line
member function means that the implicit this pointer forces the object
onto the stack (registers don't have addresses that could be
passed). Since const_iterator is now merely a thinly-disguised
pointer, that would cause terrible codegen.

So, apply a technique originally developed for QStringView and make
the operator an inline function that re-sets the member dirEntry using
a call to a QDirListing static function that takes and returns by
value. Passing by value enables passing in registers, which means the
iterator object can now be a register value and doesn't require
spilling to the stack to call the function.

Found in API-review.

Change-Id: I9f4d5be5b72249b368ef3dea9d857178ca7e16aa
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit aec8398d71d351e956683a695af51317fe65097e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/io/qdirlisting.cpp

@@ -723,14 +723,23 @@ QDirListing::const_iterator QDirListing::begin() const
 */
 
 /*!
+    \fn QDirListing::const_iterator::operator++()
+
     Advances the iterator and returns a reference to it.
 */
-QDirListing::const_iterator &QDirListing::const_iterator::operator++()
+
+/*!
+    \internal
+
+    Implements the actual advancing. Not a member function to avoid forcing
+    DirEntry objects (and therefore const_iterator ones) onto the stack.
+*/
+auto QDirListing::next(DirEntry dirEntry) -> DirEntry
 {
     dirEntry.dirListPtr->advance();
     if (!dirEntry.dirListPtr->hasIterators())
-        *this = {}; // All done, make `this` equal to the end() iterator
-    return *this;
+        return {}; // All done, make `this` equal to the end() iterator
+    return dirEntry;
 }
 
 /*!

src/corelib/io/qdirlisting.h

@@ -119,7 +119,7 @@ public:
 
         reference operator*() const { return dirEntry; }
         pointer operator->() const { return &dirEntry; }
-        Q_CORE_EXPORT const_iterator &operator++();
+        const_iterator &operator++() { dirEntry = next(dirEntry); return *this; }
         void operator++(int) { ++*this; }; // [iterator.concept.winc]/14 not required to return sth
     private:
         bool atEnd() const noexcept { return dirEntry.dirListPtr == nullptr; }
@@ -146,6 +146,8 @@ public:
 private:
     Q_DISABLE_COPY(QDirListing)
 
+    Q_CORE_EXPORT static DirEntry next(DirEntry);
+
     // Private constructor that is used in deprecated code paths.
     // `uint` instead of QDir::Filters and QDirIterator::IteratorFlags
     // because qdir.h can't be included here; qdiriterator.h can't included