[ORC] Fix -Wunused-variable in LLJIT.cpp (NFC)

llvm-project/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp:684:8:
error: unused variable 'ConcurrentCompilationSettingDefaulted' [-Werror,-Wunused-variable]
  bool ConcurrentCompilationSettingDefaulted = !SupportConcurrentCompilation;
       ^
1 error generated.

Author: DamonFool

llvm/lib/ExecutionEngine/Orc/LLJIT.cpp

@@ -681,7 +681,10 @@ Error LLJITBuilderState::prepareForConstruction() {
         inconvertibleErrorCode());
 #endif // !LLVM_ENABLE_THREADS
 
-  bool ConcurrentCompilationSettingDefaulted = !SupportConcurrentCompilation;
+  LLVM_DEBUG({
+    bool ConcurrentCompilationSettingDefaulted = !SupportConcurrentCompilation;
+  });
+
   if (!SupportConcurrentCompilation) {
 #if LLVM_ENABLE_THREADS
     SupportConcurrentCompilation = NumCompileThreads || ES || EPC;