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

llvm-svn: 218189