[DwarfDebug] Restore code that make comments stay aligned in DwarfDebug::emitDebugLocEntry

Commit 2bddab25 removed a piece of code from
DwarfDebug::emitDebugLocEntry that according to code comments
"Make sure comments stay aligned".

This patch restores that piece of code, together with the addition
of some extra checks in an existing lit test to work as a regression
test. Without this patch we incorrectly get
  .byte   159                             # 0
instead of
  .byte   159                             # DW_OP_stack_value

Differential Revision: https://reviews.llvm.org/D117441