Add test

Author: fzou1

llvm/test/LTO/X86/empty-triple.ll

@@ -0,0 +1,7 @@
+; RUN: llvm-as < %s >%t1
+; RUN: llvm-lto -o %t2 %t1  2>&1 | FileCheck --allow-empty %s
+
+; The test is to check the triple is set to default one when it's empty.
+; Otherwise, an error will be raised by llvm-lto.
+
+; CHECK-NOT: llvm-lto: error: No available targets are compatible with triple ""