[llvm] llvm-mc: Error on MCSubtargetInfo construction failure (PR #159226)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Can you add a test to llvm/test/tools/llvm-mc?
```
% llvm-mc -triple=xxx
llvm-mc: error: unable to get target for 'xxx', see --version and --triple.
```

https://github.com/llvm/llvm-project/pull/159226