use llvm_unreachable() instead of assert(0) for invalid enum values in switch statements

llvm-svn: 158542
diff --git a/lld/lib/Core/Resolver.cpp b/lld/lib/Core/Resolver.cpp
index d50a772..018cb0a 100644
--- a/lld/lib/Core/Resolver.cpp
+++ b/lld/lib/Core/Resolver.cpp
@@ -359,7 +359,7 @@
   } else if (const AbsoluteAtom* abAtom = dyn_cast<AbsoluteAtom>(&atom)) {
     _absoluteAtoms._atoms.push_back(abAtom);
   } else {
-    assert(0 && "atom has unknown definition kind");
+    llvm_unreachable("atom has unknown definition kind");
   }
 }