Fixed MSVS warning "not all control paths return a value". NFC.

llvm-svn: 256415
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 01c8110..ee04b68 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -634,6 +634,7 @@
   case InputSectionBase<ELFT>::MipsReginfo:
     return new (MReginfoSecAlloc.Allocate()) MipsReginfoOutputSection<ELFT>();
   }
+  llvm_unreachable("Unknown output section type");
 }
 
 // Create output section objects and add them to OutputSections.