Move llvm_unreachable out of switch.

This should make gcc happy and still produce a clang warning if we add
another value to the enum.

llvm-svn: 295865
diff --git a/llvm/lib/Object/ArchiveWriter.cpp b/llvm/lib/Object/ArchiveWriter.cpp
index 11cd72f..83fbf2f 100644
--- a/llvm/lib/Object/ArchiveWriter.cpp
+++ b/llvm/lib/Object/ArchiveWriter.cpp
@@ -129,9 +129,12 @@
   case object::Archive::K_BSD:
   case object::Archive::K_DARWIN:
     return true;
-  default:
-    llvm_unreachable("not supported for writting");
+  case object::Archive::K_MIPS64:
+  case object::Archive::K_DARWIN64:
+  case object::Archive::K_COFF:
+    break;
   }
+  llvm_unreachable("not supported for writting");
 }
 
 static void print32(raw_ostream &Out, object::Archive::Kind Kind,