MC: Fix MCSectionCOFF::PrintSwitchToSection

We had a few bugs:
- We were considering the GVKind instead of just looking at the section
  characteristics
- We would never print out 'y' when a section was meant to be unreadable
- We would never print out 's' when a section was meant to be shared
- We translated IMAGE_SCN_MEM_DISCARDABLE to 'n' when it should've meant
  IMAGE_SCN_LNK_REMOVE

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218189 91177308-0d34-0410-b5e6-96231b3b80d8