Make AsmStreamer maintain a notion of the current section, pushing it up from the

Make AsmStreamer maintain a notion of the current section, pushing it up from the
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