[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
diff --git a/lld/ELF/OutputSections.cpp b/lld/ELF/OutputSections.cpp
index af96052..1901af2 100644
--- a/lld/ELF/OutputSections.cpp
+++ b/lld/ELF/OutputSections.cpp
@@ -87,7 +87,6 @@
 template <class ELFT> void OutputSection::finalize() {
   if ((this->Flags & SHF_LINK_ORDER) && !this->Sections.empty()) {
     std::sort(Sections.begin(), Sections.end(), compareByFilePosition<ELFT>);
-    Size = 0;
     assignOffsets<ELFT>();
 
     // We must preserve the link order dependency of sections with the
@@ -133,7 +132,7 @@
 // This function is called after we sort input sections
 // and scan relocations to setup sections' offsets.
 template <class ELFT> void OutputSection::assignOffsets() {
-  uint64_t Off = this->Size;
+  uint64_t Off = 0;
   for (InputSection *S : Sections) {
     Off = alignTo(Off, S->Alignment);
     S->OutSecOff = Off;