This adds the --debug-vars option to llvm-objdump, which prints
locations (registers/memory) of source-level variables alongside the
disassembly based on DWARF info. A vertical line is printed for each
live-range, with a label at the top giving the variable name and
location, and the position and length of the line indicating the program
counter range in which it is valid.
Differential revision: https://reviews.llvm.org/D70720