[llvm-jitlink] Prevent dead-stripping of test callback

`llvm_jitlink_setTestResultOverride` is used via runtime lookup by
tests, so make sure it is not dead-stripped from llvm-jitlink in release
builds.

Fixes llvm#53203

Differential Revision: https://reviews.llvm.org/D117609

Author: benlangmuir

llvm/tools/llvm-jitlink/llvm-jitlink.cpp

@@ -231,7 +231,8 @@ LLVM_ATTRIBUTE_USED void linkComponents() {
 static bool UseTestResultOverride = false;
 static int64_t TestResultOverride = 0;
 
-extern "C" void llvm_jitlink_setTestResultOverride(int64_t Value) {
+extern "C" LLVM_ATTRIBUTE_USED void
+llvm_jitlink_setTestResultOverride(int64_t Value) {
   TestResultOverride = Value;
   UseTestResultOverride = true;
 }