[ELF] - Reset output section size when assigning offsets.

In many places we reset Size to 0 before calling assignOffsets()
manually. Sometimes we don't do that. 
It looks we can just always do that inside.

Previous code had:
template <class ELFT> void OutputSection::assignOffsets() {
  uint64_t Off = Size;

And tests feels fine with Off = 0. 
I think Off = Size make no sence.

Differential revision: https://reviews.llvm.org/D30463

llvm-svn: 296609

lld/ELF/Writer.cpp

@@ -1025,7 +1025,6 @@ static void finalizeSynthetic(const std::vector<SyntheticSection *> &Sections) {
   for (SyntheticSection *SS : Sections)
     if (SS && SS->OutSec && !SS->empty()) {
       SS->finalizeContents();
-      SS->OutSec->Size = 0;
       SS->OutSec->template assignOffsets<ELFT>();
     }
 }