[llvm] Add format check for MCSubtargetFeatures (#180943)
`SubtargetFeatures` class has next constraints:
https://github.com/llvm/llvm-project/blob/c9d065abc15846deb95a23fb0b3e1855d3d26314/llvm/include/llvm/TargetParser/SubtargetFeature.h#L167-L174
At this moment feature string isn't checked for fitting in such format.
This leads to assertion failure, for example in lldb:
https://github.com/llvm/llvm-project/pull/180901, when features from
user's input don't meet the requirements.
With implementing additional format check we can avoid such problems.
