Mark function as 'used' so that LTO doesn't try to get rid of it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165777 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/unittests/ExecutionEngine/JIT/JITTest.cpp b/unittests/ExecutionEngine/JIT/JITTest.cpp
index 5e2af03..1eff4d6 100644
--- a/unittests/ExecutionEngine/JIT/JITTest.cpp
+++ b/unittests/ExecutionEngine/JIT/JITTest.cpp
@@ -633,7 +633,7 @@
 // This function is intentionally defined differently in the statically-compiled
 // program from the IR input to the JIT to assert that the JIT doesn't use its
 // definition.
-extern "C" int32_t JITTest_AvailableExternallyFunction() {
+extern "C" int32_t JITTest_AvailableExternallyFunction() __attribute__((used)) {
   return 42;
 }
 namespace {