llvm-cov: Fix a misuse of ArrayRef::slice I introduced in r217430

It appears this code was completely untested, so using ArrayRef wrong
didn't break anything obvious.

llvm-svn: 217476

llvm/tools/llvm-cov/SourceCoverageView.cpp

@@ -206,7 +206,7 @@ gatherLineSubViews(size_t &CurrentIdx,
   while (CurrentIdx < E &&
          Items[CurrentIdx]->getSubViewsExpansionLine() == LineNo)
     ++CurrentIdx;
-  return Items.slice(PrevIdx, CurrentIdx);
+  return Items.slice(PrevIdx, CurrentIdx - PrevIdx);
 }
 
 void SourceCoverageView::render(raw_ostream &OS, unsigned Offset) {