MC: Add dyn_cast support to MCSection.

MC: Add dyn_cast support to MCSection.
 - Of questionable utility, since in general anything which wants to do this should probably be within a target specific hook, which can rely on the sections being of the appropriate type. However, it can be useful for short term hacks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103980 91177308-0d34-0410-b5e6-96231b3b80d8