[llvm] [TableGen] Print a warning when a Processor contains duplicate Features / TuneFeatures (PR #137864)

[Jay Foad via llvm-commits]

jayfoad wrote:

This is printing new warnings when I build `llc`:
```
[1938/2915] Building PPCGenSubtargetInfo.inc...
warning: Processor future contains duplicate feature 'predictable-select-expensive'
warning: Processor pwr10 contains duplicate feature 'predictable-select-expensive'
warning: Processor pwr11 contains duplicate feature 'predictable-select-expensive'
warning: Processor pwr9 contains duplicate feature 'predictable-select-expensive'
```

https://github.com/llvm/llvm-project/pull/137864