[llvm-mc] Pushing plumbing through for --fatal-warnings flag.

llvm-svn: 243334
diff --git a/llvm/test/tools/llvm-mc/fatal_warnings.test b/llvm/test/tools/llvm-mc/fatal_warnings.test
new file mode 100644
index 0000000..df73ad8
--- /dev/null
+++ b/llvm/test/tools/llvm-mc/fatal_warnings.test
@@ -0,0 +1,5 @@
+# RUN: not llvm-mc --fatal-warnings %s 2>&1 | FileCheck %s
+# XFAIL: hexagon
+
+# CHECK: error: .warning directive invoked in source file
+.warning