[CodeGen] Print user-friendly debug locations as MI comments

If available, print the file, line and column of the DebugLoc attached
to the MachineInstr:

MOV16mr $rbp, 1, $noreg, -112, $noreg, killed renamable $ax, debug-location !56 :: (store 2 into %ir.._value12); stepping.swift:10:17
renamable $edx = MOVZX32rm16 $rbp, 1, $noreg, -112, $noreg, debug-location !62 :: (dereferenceable load 2 from %ir.._value13); stepping.swift:10:17

Differential Revision: https://reviews.llvm.org/D45992

llvm-svn: 330709