I tested an upgrade today from Ubuntu 16.04 to Ubuntu 18.04 using the dist-upgrader from -proposed but not using the clone attached in this bug report and the upgrade did complete successfully with libc6 being upgraded first.

Log started: 2018-08-09  09:56:06
(Reading database ... ^M(Reading database ... 5%^M(Reading database ... 10%^M(Reading database ... 15%^M(Reading database ... 20%^M(Reading database ... 25%^M(Reading database ... 30%^M(Reading database ... 35%^M(Reading database ... 40%^M(Reading database ... 45%^M(Reading database ... 50%^M(Reading database ... 55%^M(Reading database ... 60%^M(Reading database ... 65%^M(Reading database ... 70%^M(Reading database ... 75%^M(Reading database ... 80%^M(Reading database ... 85%^M(Reading database ... 90%^M(Reading database ... 95%^M(Reading database ... 100%^M(Reading database ... 278731 files and directories currently installed.)^M
Preparing to unpack .../libc-dev-bin_2.27-3ubuntu1_amd64.deb ...^M
Unpacking libc-dev-bin (2.27-3ubuntu1) over (2.23-0ubuntu10) ...^M
Preparing to unpack .../libc6-dev_2.27-3ubuntu1_amd64.deb ...^M
Unpacking libc6-dev:amd64 (2.27-3ubuntu1) over (2.23-0ubuntu10) ...^M
Preparing to unpack .../libc6-dbg_2.27-3ubuntu1_amd64.deb ...^M
Unpacking libc6-dbg:amd64 (2.27-3ubuntu1) over (2.23-0ubuntu10) ...^M
Preparing to unpack .../locales_2.27-3ubuntu1_all.deb ...^M
Unpacking locales (2.27-3ubuntu1) over (2.23-0ubuntu10) ...^M
Preparing to unpack .../libc6_2.27-3ubuntu1_amd64.deb ...^M
Checking for services that may need to be restarted...^M
Checking init scripts...^M
Nothing to restart.^M
Unpacking libc6:amd64 (2.27-3ubuntu1) over (2.23-0ubuntu10) ...^M
Setting up libc6:amd64 (2.27-3ubuntu1) ...^M
Installing new version of config file /etc/ld.so.conf.d/x86_64-linux-gnu.conf ...^M
Checking for services that may need to be restarted...^M
Checking init scripts...^M
Nothing to restart.^M
Processing triggers for man-db (2.7.5-1) ...^M
(Reading database ... ^M(Reading database ... 5%^M(Reading database ... 10%^M(Reading database ... 15%^M(Reading database ... 20%^M(Reading database ... 25%^M(Reading database ... 30%^M(Reading database ... 35%^M(Reading database ... 40%^M(Reading database ... 45%^M(Reading database ... 50%^M(Reading database ... 55%^M(Reading database ... 60%^M(Reading database ... 65%^M(Reading database ... 70%^M(Reading database ... 75%^M(Reading database ... 80%^M(Reading database ... 85%^M(Reading database ... 90%^M(Reading database ... 95%^M(Reading database ... 100%^M(Reading database ... 278807 files and directories currently installed.)^M
Preparing to unpack .../libc-bin_2.27-3ubuntu1_amd64.deb ...^M
Unpacking libc-bin (2.27-3ubuntu1) over (2.23-0ubuntu10) ...^M
Processing triggers for man-db (2.7.5-1) ...^M
Setting up libc-bin (2.27-3ubuntu1) ...^M
Setting up libc-dev-bin (2.27-3ubuntu1) ...^M
Setting up libc6-dev:amd64 (2.27-3ubuntu1) ...^M
Setting up libc6-dbg:amd64 (2.27-3ubuntu1) ...^M
Setting up locales (2.27-3ubuntu1) ...^M
Installing new version of config file /etc/locale.alias ...^M
Generating locales (this might take a while)...^M
...
Generation complete.^M
Log ended: 2018-08-09  09:56:24

Log started: 2018-08-09  09:57:01
(Reading database ... ^M(Reading database ... 5%^M(Reading database ... 10%^M(Reading database ... 15%^M(Reading database ... 20%^M(Reading database ... 25%^M(Reading database ... 30%^M(Reading database ... 35%^M(Reading database ... 40%^M(Reading database ... 45%^M(Reading database ... 50%^M(Reading database ... 55%^M(Reading database ... 60%^M(Reading database ... 65%^M(Reading database ... 70%^M(Reading database ... 75%^M(Reading database ... 80%^M(Reading database ... 85%^M(Reading database ... 90%^M(Reading database ... 95%^M(Reading database ... 100%^M(Reading database ... 278807 files and directories currently installed.)^M
Preparing to unpack .../base-files_10.1ubuntu2.1_amd64.deb ...^M
Unpacking base-files (10.1ubuntu2.1) over (9.4ubuntu4.7) ...^M
Processing triggers for plymouth-theme-ubuntu-text (0.9.2-3ubuntu13.5) ...^M
...