MC: Only allow changing feature bits in MCSubtargetInfo

MC: Only allow changing feature bits in MCSubtargetInfo

Disallow all mutation of `MCSubtargetInfo` expect the feature bits.

Besides deleting the assignment operators -- which were dead "code" --
this restricts `InitMCProcessorInfo()` to subclass initialization
sequences, and exposes a new more limited function called
`setDefaultFeatures()` for use by the ARMAsmParser `.cpu` directive.

There's a small functional change here: ARMAsmParser used to adjust
`MCSubtargetInfo::CPUSchedModel` as a side effect of calling
`InitMCProcessorInfo()`, but I've removed that suspicious behaviour.
Since the AsmParser shouldn't be doing any scheduling, there shouldn't
be any observable change...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241961 91177308-0d34-0410-b5e6-96231b3b80d8