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.

llvm-svn: 79324