change AsmPrinter to switch sections using AsmStreamer instead of

change AsmPrinter to switch sections using AsmStreamer instead of
doing it directly.  This requires const'izing a bunch of stuff that
took sections, but this seems like the right semantic thing to do:
emitting a label to a section shouldn't mutate the MCSection object
itself, for example.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79227 91177308-0d34-0410-b5e6-96231b3b80d8