implement assignment in only one place

... and comment on what to do if we really wanted a copy-swap-idiom.
The need to doc comment about not to use assign() after default ctor is
also gone with the temporary swap.

Change-Id: I2a49091b2a41cf155e912e3c373dbbe81c7f9737

sc/inc/cellvalue.hxx

@@ -123,10 +123,6 @@ struct SC_DLLPUBLIC ScRefCellValue
 
     /**
      * Take cell value from specified position in specified document.
-     *
-     * Avoid the sequence of ScRefCellValue() default ctor followed by assign()
-     * as it results in performance penalty, use the
-     * ScRefCellValue(ScDocument&,const ScAddress&) ctor instead.
      */
     void assign( ScDocument& rDoc, const ScAddress& rPos );
 

sc/source/core/data/cellvalue.cxx

@@ -495,9 +495,7 @@ ScRefCellValue::ScRefCellValue( const ScRefCellValue& r ) : meType(r.meType), mf
 
 ScRefCellValue::ScRefCellValue( ScDocument& rDoc, const ScAddress& rPos )
 {
-    const ScRefCellValue& rCell = rDoc.GetRefCellValue(rPos);
-    meType = rCell.meType;
-    mfValue = rCell.mfValue;
+    assign( rDoc, rPos);
 }
 
 ScRefCellValue::~ScRefCellValue()
@@ -593,6 +591,11 @@ bool ScRefCellValue::equalsWithoutFormat( const ScRefCellValue& r ) const
 
 ScRefCellValue& ScRefCellValue::operator= ( const ScRefCellValue& r )
 {
+    // So we *could* have a copy-swap-idiom here for exception-safety if we had
+    // to slow down things.. but then implement an explicit move-ctor and pass
+    // r by-value instead of manually creating a temporary so the compiler can
+    // take advantage. And initialize
+    // ScRefCellValue(ScDocument&,const ScAddress&) with default ctor.
     meType = r.meType;
     mfValue = r.mfValue;    // largest member of union
     return *this;