This is the clang side of the RFC in http://lists.llvm.org/pipermail/cfe-dev/2017-February/052549.html
Note that in contrast to the original suggestion -fsource-asm here we use the preferred -fverbose-asm. Basically explicitly saying -fverbose-asm in the command line enables a minimum amount of debugging, so in AsmPrinter we can use it to print the source code.
This patch introduces a -masm-source flag for cc1 that maps to the AsmSource value in the llvm code generation.
See the llvm side of this in https://reviews.llvm.org/D30897