Replace unreachable code with llvm_unreachable.

llvm-svn: 236261
diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 2ace984..d11270b 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -1338,7 +1338,7 @@
     break;
 
   case ELF::SHT_DYNAMIC:
-    sh_link = ShStrTabBuilder.getOffset(Section.getSectionName());
+    llvm_unreachable("SHT_DYNAMIC in a relocatable object");
     break;
 
   case ELF::SHT_REL: