Re-implement DebugIR in a way that does not subclass AssemblyWriter:
- move AsmWriter.h from public headers into lib
- marked all AssemblyWriter functions as non-virtual; no need to override them
- DebugIR now "plugs into" AssemblyWriter with an AssemblyAnnotationWriter helper
- exposed flags to control hiding of a) debug metadata b) debug intrinsic calls

C/R: Paul Redmond



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182617 91177308-0d34-0410-b5e6-96231b3b80d8