1. Make MCSection an abstract class.

1. Make MCSection an abstract class.
2. Move section switch printing to MCSection virtual method which takes a
   TAI.  This eliminates textual formatting stuff from TLOF.
3. Eliminate SwitchToSectionDirective, getSectionFlagsAsString, and
   TLOFELF::AtIsCommentChar.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78510 91177308-0d34-0410-b5e6-96231b3b80d8