Latest versions of Ubuntu packages cause hard to fix conflicts (1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56)

This is the error I am getting:

```
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
The following packages will be REMOVED:
libc++1-21 libc++abi1-21
The following NEW packages will be installed:
libc++1 libc++abi1
The following packages will be upgraded:
clang-21 clang-format-21 clang-tidy-21 clang-tools-21 clangd-21 libc++-21-dev libc++abi-21-dev libclang-common-21-dev libclang-cpp21 libclang-rt-21-dev libclang1-21 libllvm21 lld-21 llvm-21 llvm-21-linker-tools
llvm-21-runtime
16 upgraded, 2 newly installed, 2 to remove and 0 not upgraded.
Need to get 99.7 MB of archives.
After this operation, 709 kB disk space will be freed.
Do you want to continue? [Y/n] y
Get:1 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 libc++-21-dev amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [1567 kB]
Get:2 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 libc++abi-21-dev amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [146 kB]
Get:3 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 libc++abi1 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [120 kB]
Get:4 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 libc++1 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [368 kB]
Get:5 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 lld-21 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [1578 kB]
Get:6 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 clang-tidy-21 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [2127 kB]
Get:7 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 clang-tools-21 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [9605 kB]
Get:8 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 clang-21 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [176 kB]
Get:9 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 clangd-21 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [3591 kB]
Get:10 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 libclang-common-21-dev amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [801 kB]
Get:11 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 clang-format-21 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [104 kB]
Get:12 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 libclang-cpp21 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [13.9 MB]
Get:13 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 libllvm21 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [30.9 MB]
Get:14 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 libclang1-21 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [8239 kB]
Get:15 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 llvm-21 amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [20.1 MB]
Get:16 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 llvm-21-runtime amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [611 kB]
Get:17 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 llvm-21-linker-tools amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [1384 kB]
Get:18 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 libclang-rt-21-dev amd64 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 [4304 kB]
Fetched 99.7 MB in 15s (6500 kB/s)
(Reading database ... 105254 files and directories currently installed.)
Preparing to unpack .../libc++-21-dev_1%3a21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56_amd64.deb ...
Unpacking libc++-21-dev (1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56) over (1:21.1.5~++20251023083335+45afac62e373-1~exp1~20251023083454.54) ...
dpkg: error processing archive /var/cache/apt/archives/libc++-21-dev_1%3a21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56_amd64.deb (--unpack):
trying to overwrite '/usr/lib/llvm-21/lib/libc++.so.1', which is also in package libc++1-21 1:21.1.5~++20251023083335+45afac62e373-1~exp1~20251023083454.54
Preparing to unpack .../libc++abi-21-dev_1%3a21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56_amd64.deb ...
Unpacking libc++abi-21-dev (1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56) over (1:21.1.5~++20251023083335+45afac62e373-1~exp1~20251023083454.54) ...
dpkg: error processing archive /var/cache/apt/archives/libc++abi-21-dev_1%3a21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56_amd64.deb (--unpack):
trying to overwrite '/usr/lib/llvm-21/lib/libc++abi.so.1', which is also in package libc++abi1-21 1:21.1.5~++20251023083335+45afac62e373-1~exp1~20251023083454.54
Errors were encountered while processing:
/var/cache/apt/archives/libc++-21-dev_1%3a21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56_amd64.deb
/var/cache/apt/archives/libc++abi-21-dev_1%3a21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56_amd64.deb
needrestart is being skipped since dpkg has failed
E: Sub-process /usr/bin/dpkg returned an error code (1)
```

I was trying to report this via salsa.debian.org, but unfortunately it's going to take some time to get approved (if at all).

Perhaps also of interest:

```
$ apt policy libc++1 libc++abi1
libc++1:
Installed: (none)
Candidate: 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56
Version table:
1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 900
900 http://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 Packages
1:20.1.8~++20250804090239+87f0227cb601-1~exp1~20250804210352.139 900
900 http://apt.llvm.org/noble llvm-toolchain-noble-20/main amd64 Packages
1:18.0-59~exp2 500
500 http://archive.ubuntu.com/ubuntu noble/universe amd64 Packages
libc++abi1:
Installed: (none)
Candidate: 1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56
Version table:
1:21.1.6~++20251111103412+4b2ac3f7a210-1~exp1~20251111103428.56 900
900 http://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 Packages
1:20.1.8~++20250804090239+87f0227cb601-1~exp1~20250804210352.139 900
900 http://apt.llvm.org/noble llvm-toolchain-noble-20/main amd64 Packages
1:18.0-59~exp2 500
500 http://archive.ubuntu.com/ubuntu noble/universe amd64 Packages
```

FYI: @sylvestre (since you seem to be _the_ maintainer?!)

Reproduce the reported upgrade on Ubuntu Noble using the LLVM 21 packages and the versions shown in the error. Inspect package ownership for libc++-21-dev, libc++abi-21-dev, libc++1-21, and libc++abi1-21; done means the upgrade completes without dpkg overwrite conflicts.

Written by the indexing model from the issue text.