MCAsmStreamer. Based on this, eliminate the current section from AsmPrinter.
While I'm at it, clean up the last of the horrible "switch to null section" stuff
and add an assert. This change is in preparation for completely eliminating
asmprinter::switchtosection.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79324 91177308-0d34-0410-b5e6-96231b3b80d8