PassWrapper: use non-deprecated lookupTarget method

This avoids an extra trip through a triple string by directly passing
the Triple, and has been available since LLVM 21. The string overload
was deprecated today and throws an error on our CI for HEAD due to
-Werror paranoia, so we may as well clean this up now and also skip the
conversion on LLVM 21 since we can.

@rustbot label llvm-main

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -283,7 +283,11 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
   std::string Error;
   auto Trip = Triple(Triple::normalize(TripleStr));
   const llvm::Target *TheTarget =
+#if LLVM_VERSION_GE(21, 0)
+      TargetRegistry::lookupTarget(Trip, Error);
+#else
       TargetRegistry::lookupTarget(Trip.getTriple(), Error);
+#endif
   if (TheTarget == nullptr) {
     LLVMRustSetLastError(Error.c_str());
     return nullptr;