[DebugInfo] Align comments in debug_loc section

Summary:
This commit updates the BufferByteStreamer, used by DebugLocStream
to buffer bytes/comments to put in the debug_loc section, to
make sure that the Buffer and Comments vectors are synced.
Previously, when an SLEB128 or ULEB128 was emitted together with
a comment, the vectors could be out-of-sync if the LEB encoding
added several entries to the Buffer vectors, while we only added
a single entry to the Comments vector.

The goal with this is to get the comments in the debug_loc
section in the .s file correctly aligned.

Example (using ARM as target):
Instead of

  .byte 144                     @ sub-register DW_OP_regx
  .byte 128                     @ 256
  .byte 2                       @ DW_OP_piece
  .byte 147                     @ 8
  .byte 8                       @ sub-register DW_OP_regx
  .byte 144                     @ 257
  .byte 129                     @ DW_OP_piece
  .byte 2                       @ 8
  .byte 147                     @
  .byte 8                       @

we now get

  .byte 144                     @ sub-register DW_OP_regx
  .byte 128                     @ 256
  .byte 2                       @
  .byte 147                     @ DW_OP_piece
  .byte 8                       @ 8
  .byte 144                     @ sub-register DW_OP_regx
  .byte 129                     @ 257
  .byte 2                       @
  .byte 147                     @ DW_OP_piece
  .byte 8                       @ 8

Reviewers: JDevlieghere, rnk, aprantl

Reviewed By: aprantl

Subscribers: davide, Ka-Ka, uabelho, aemerson, javed.absar, kristof.beyls, llvm-commits, JDevlieghere

Differential Revision: https://reviews.llvm.org/D41763

llvm-svn: 321907