Patch by Ray Donnelly to print register names instead of numbers.
Do not commit until Ray approves!
This patch doesn't work right. I am seeing stuff like this in the emitted code:
pushq %rbp .seh_pushreg %rdi
Which, I guess, makes sense, because MCAsmStreamer::EmitRegisterName performs DWARF->LLVM register index mapping before emitting the name.
If you are gonna try to fix this, please not that .seh_... directives seem to use "SEH" register numbers, which are probably different from normal LLVM register indices.