Conditional usage of LLVM DebugFlag

DebugFlag is conditionally exported by LLVM in llvm/Support/Debug.h
in-between an #ifndef NDEBUG block; RustWrapper should not
unconditionally use it. This closes #3701.

Signed-off-by: Luca Bruno <lucab@debian.org>

src/rustllvm/RustWrapper.cpp

@@ -493,5 +493,7 @@ extern "C" LLVMValueRef LLVMBuildAtomicRMW(LLVMBuilderRef B,
 }
 
 extern "C" void LLVMSetDebug(int Enabled) {
+#ifndef NDEBUG
   DebugFlag = Enabled;
+#endif
 }