Auto merge of #115079 - cuviper:unused-mcinfo, r=Mark-Simulacrum

Move a local to the `#if` block where it is used

For other cases (LLVM < 17), this was complaining under `-Wall`:

```
warning: llvm-wrapper/PassWrapper.cpp: In function ‘void LLVMRustPrintTargetCPUs(LLVMTargetMachineRef, const char*)’:
warning: llvm-wrapper/PassWrapper.cpp:311:26: warning: unused variable ‘MCInfo’ [-Wunused-variable]
warning:   311 |   const MCSubtargetInfo *MCInfo = Target->getMCSubtargetInfo();
warning:       |                          ^~~~~~
```

Author: bors