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

[Min-Yih Hsu via llvm-commits]

mshockwave 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'
> ```

Pending #137903 

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