[ELF] Use stable_sort for compareByFilePosition

The comment at the top of compareByFilePosition indicates that it relies
on stable_sort to preserve the order of synthetic sections. We were
using sort instead of stable_sort, however, leading to incorrect
synthetic section ordering.

Differential Revision: https://reviews.llvm.org/D35473

llvm-svn: 308207

lld/ELF/LinkerScript.cpp

@@ -1084,7 +1084,7 @@ template <class ELFT> void OutputSectionCommand::finalize() {
       }
 
   if ((Sec->Flags & SHF_LINK_ORDER)) {
-    std::sort(Sections.begin(), Sections.end(), compareByFilePosition);
+    std::stable_sort(Sections.begin(), Sections.end(), compareByFilePosition);
     for (int I = 0, N = Sections.size(); I < N; ++I)
       *ScriptSections[I] = Sections[I];