Pull up AssemblyWriter interface into header to allow subclassing
- made all functions virtual so that subclasses can specialize them
- add printInstructionLine so that subclasses can choose whether or not to
print the newline character (without having to implement printBasicBlock()
- added a second constructor to AssemblyWriter that does not require a
SlotTracker, as required in order to keep the SlotTracker helper class outside
AsmWriter.h and buried in the implementation.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181466
91177308-0d34-0410-b5e6-
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