[llvm-ar] Added llvm_unreachable to quiet -Wreturn-type warnings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185751 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/llvm-ar/llvm-ar.cpp b/tools/llvm-ar/llvm-ar.cpp
index 515f899..a9297eb 100644
--- a/tools/llvm-ar/llvm-ar.cpp
+++ b/tools/llvm-ar/llvm-ar.cpp
@@ -655,6 +655,8 @@
   case ReplaceOrInsert:
     return true;
   }
+
+  llvm_unreachable("Missing entry in covered switch.");
 }
 
 // main - main program for llvm-ar .. see comments in the code