This is the LLVM side of the proposal in http://lists.llvm.org/pipermail/cfe-dev/2017-February/052549.html
Basically introduces a new AsmPrinterHandler that is enabled when AsmSource is not None. AsmSource is not just a boolean because we do not want to clutter the assembler with debug directives if we are just using -fverbose-asm (and no other debug information has been requested).
See the clang side of this in https://reviews.llvm.org/D30898
*TODO*: Add tests