Use OutputSectionBase in a few cases where we don't need a OutputSection.

NFC. This is just preparation for adding a new OutputSection dedicated to
SHF_MERGE input sections.

llvm-svn: 250419
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 0a86a23..bd82c69 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -427,7 +427,7 @@
       Map.lookup({".fini_array", SHT_FINI_ARRAY, SHF_WRITE | SHF_ALLOC});
 
   auto AddStartEnd = [&](StringRef Start, StringRef End,
-                         OutputSection<ELFT> *OS) {
+                         OutputSectionBase<ELFT::Is64Bits> *OS) {
     if (OS) {
       Symtab.addSyntheticSym(Start, *OS, 0);
       Symtab.addSyntheticSym(End, *OS, OS->getSize());