Address Ried's post-commit review comment on r351477.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351493 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/llvm/Demangle/DemangleConfig.h b/include/llvm/Demangle/DemangleConfig.h
index f886188..528f538 100644
--- a/include/llvm/Demangle/DemangleConfig.h
+++ b/include/llvm/Demangle/DemangleConfig.h
@@ -62,6 +62,8 @@
 #define DEMANGLE_UNREACHABLE __builtin_unreachable()
 #elif defined(_MSC_VER)
 #define DEMANGLE_UNREACHABLE __assume(false)
+#else
+#define DEMANGLE_UNREACHABLE
 #endif
 
 #if __has_attribute(noinline) || DEMANGLE_GNUC_PREREQ(3, 4, 0)