[ELF] - replace error() with llvm_unreachable. 

llvm-svn: 276790
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 904e94f..f3a3bbc 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -775,7 +775,7 @@
       return B->getVA<ELF64BE>();
     break;
   default:
-    fatal("unsupported target");
+    llvm_unreachable("unsupported target");
   }
   error("symbol not found: " + S);
   return 0;