Silence a "not all control paths return a value" warning; NFC.

llvm-svn: 245562
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index f124c22..8be92fe 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -3063,7 +3063,7 @@
   case llvm::Reloc::DynamicNoPIC:
     return "dynamic-no-pic";
   }
-  assert(false && "Unknown Reloc::Model kind");
+  llvm_unreachable("Unknown Reloc::Model kind");
 }
 
 static void AddAssemblerKPIC(const ToolChain &ToolChain, const ArgList &Args,