[llvm] [llvm] Add format check for MCSubtargetFeatures (PR #180943)

[Georgiy Samoylov via llvm-commits]

sga-sc wrote:

@DavidSpickett all such strings that come after `// Invalid Strings` are strings that violate feature format which I referenced in comment here: https://github.com/llvm/llvm-project/pull/184355. 
To be honest, I don't understand what you offer to change in comment in that PR. 

https://github.com/llvm/llvm-project/pull/180943