Add some scaffolding for a new experimental asmprinter

Add some scaffolding for a new experimental asmprinter
implementation.  The idea is that we want asmprinting to
work by converting MachineInstrs into a new MCInst class,
then the per-instruction asmprinter works on MCInst.  MCInst
and the new asmprinters will not depend on most of the
llvm code generators.  This allows building diassemblers
that don't link in the whole llvm code generator.  This is
step #1 of many.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73743 91177308-0d34-0410-b5e6-96231b3b80d8