Remove redundant -Zdebug-llvm option

The same effect can be achieved using -Cllvm-args=-debug

Refs #46437 as it removes LLVMRustSetDebug()

src/rustllvm/RustWrapper.cpp

@@ -384,12 +384,6 @@ LLVMRustBuildAtomicFence(LLVMBuilderRef B, LLVMAtomicOrdering Order,
   return wrap(unwrap(B)->CreateFence(fromRust(Order), fromRust(Scope)));
 }
 
-extern "C" void LLVMRustSetDebug(int Enabled) {
-#ifndef NDEBUG
-  DebugFlag = Enabled;
-#endif
-}
-
 enum class LLVMRustAsmDialect {
   Other,
   Att,