MC: Constify MCSubtargetInfo in getDeprecationInfo(), NFC

There's no reason to be able to mutate `MCSubtargetInfo` in
`getDeprecationInfo()`.  Constify the reference.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241693 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed