QRingBuffer: restrict QRingChunk::toByteArray() calls to rvalue *this

It's only user, QRingBuffer::read(), calls it on an rvalue, so we
don't need the lvalue overload, and we can devil-may-care-like just
std::move(chunk) in the non-isShared() case.

Change-Id: I99c16862f5586125c6346ce5f969dc735de738b8
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/tools/qringbuffer.cpp

@@ -62,7 +62,7 @@ void QRingChunk::detach()
     tailOffset = chunkSize;
 }
 
-QByteArray QRingChunk::toByteArray()
+QByteArray QRingChunk::toByteArray() &&
 {
     if (headOffset != 0 || tailOffset != chunk.size()) {
         if (isShared())
@@ -79,7 +79,7 @@ QByteArray QRingChunk::toByteArray()
         chunk.resize(tailOffset);
     }
 
-    return chunk;
+    return std::move(chunk);
 }
 
 /*!

src/corelib/tools/qringbuffer_p.h

@@ -114,7 +114,7 @@ public:
         return !chunk.isDetached();
     }
     Q_CORE_EXPORT void detach();
-    QByteArray toByteArray();
+    QByteArray toByteArray() &&;
 
     // getters
     inline qsizetype head() const