[Support] Speedup llvm-dwarfdump 3.9x

Currently `strace llvm-dwarfdump x.debug >/tmp/file`:

  ioctl(1, TCGETS, 0x7ffd64d7f340)        = -1 ENOTTY (Inappropriate ioctl for device)
  write(1, "           DW_AT_decl_line\t(89)\n"..., 4096) = 4096
  ioctl(1, TCGETS, 0x7ffd64d7f400)        = -1 ENOTTY (Inappropriate ioctl for device)
  ioctl(1, TCGETS, 0x7ffd64d7f410)        = -1 ENOTTY (Inappropriate ioctl for device)
  ioctl(1, TCGETS, 0x7ffd64d7f400)        = -1 ENOTTY (Inappropriate ioctl for device)

After this patch:

  write(1, "0000000000001102 \"strlen\")\n     "..., 4096) = 4096
  write(1, "site\n                  DW_AT_low"..., 4096) = 4096
  write(1, "d53)\n\n0x000e4d4d:       DW_TAG_G"..., 4096) = 4096

The same speedup can be achieved by `--color=0` but that is not much convenient.

This implementation has been suggested by Joerg Sonnenberger.

Differential Revision: https://reviews.llvm.org/D86406