[FFe] update llvm-defaults toolchain to 11

[Dimitri John Ledkov]

llvm-toolchain-11 was delayed a bit this cycle I didn't land earlier.

llvm-toolchain is a fast moving package, and was anticipated to be available and the default.

It is desired for defaults to point to the latest stable llvm-toolchain, even if very few packages use it in the archive.

$ reverse-depends -c main -b src:llvm-defaults
Reverse-Testsuite-Triggers
* boost1.71 (for clang)
* libreoffice (for clang)
* squid (for clang)

Reverse-Build-Depends
* ceph (for clang)
* libreoffice (for clang)
* mozjs78 (for llvm)
* mozjs78 (for clang)
* squid (for clang)
* v4l-utils (for clang)

If this doesn't land well, we can revert it.

[Steve Langasek (vorlon)]

Reverse-Depends
* arcanist-clang-format-linter
* cargo (for clang)

What's the impact on rust ecosystem buildability from updating clang?

> If this doesn't land well, we can revert it.

We also have the option of updating any failing packages to hard-code an earlier version of llvm, instead.

So I'm broadly +1 but would like to have more surety regarding the impact on archive buildability, given the short window before release.

[Launchpad Janitor (janitor)]

[Expired for llvm-defaults (Ubuntu) because there has been no activity for 60 days.]