This is simply the result of:
cp -r pkgs/development/compilers/llvm/10 pkgs/development/compilers/llvm/11/

(cherry picked from commit 28f2797)

http://lists.llvm.org/pipermail/release-testers/2020-July/001305.html

TODO: Enable Polly.

Co-authored-by: conferno <conferno@camfex.cz>
(cherry picked from commit 0a8334d)

http://lists.llvm.org/pipermail/release-testers/2020-August/001321.html
(cherry picked from commit df267a4)

Ok, so I was about to update the patch (didn't apply anymore) when I
also started looking at it's usage and realized that
NIX_CHROMIUM_PLUGIN_PATH_ (and other substrings) only appears in the
patch itself (i.e. it seemed like we don't need this patch anymore).

Turns out that we have this patch since 2014 (1b84fbf) and it was
only ever used for NIX_CHROMIUM_PLUGIN_PATH_WIDEVINE (and from the log
it isn't clear if/when or how well that worked). But in 2019 that last
usage got removed (545d58a) so we should be able to safely drop this
patch now :) \o/

(I just wanted to note that as it seemed somewhat of a funny story :D
But there is of course nothing wrong with it.)
Git history (git log --oneline -S NIX_CHROMIUM_PLUGIN_PATH_):
7205bd6 ungoogled-chromium: init at 81.0.4044.92-2
545d58a chromium: fix widevine
cd3283f chromium: 67.0.3396.99 -> 68.0.3440.75
72d7b5d chromium: fix nix_plugin_paths for 68+
7a3a16d chromium: Remove plugin paths patch for version 50
79d18eb chromium: Update dev channel to v52.0.2743.10
c7a3645 chromium: Remove stuff for versions <= v51
8b97ca2 chromium: Update all channels to latest versions
b9093f1 chromium: Updates, fixes NixOS#11492
471cdd1 chromium: Update beta and dev channels.
5c6aa39 chromium: Cleanup old patch and update stable
af54ddf chromium: Drop plugin_paths patch for old versions.
6a8afa4 chromium: Fix plugin_paths patch for version 44.
0aad4b7 chromium: Update all channels to latest versions.
1b84fbf chromium: Allow env vars for passing plugin paths.

(cherry picked from commit 2213c46)

I didn't look into this yet but IIRC M86 will finally have a flag for
Linux to enable VA-API. So we shouldn't need
enable-video-acceleration-on-linux.patch anymore.

But we likely need to update enable-vdpau-support-for-nvidia.patch
when/before M86 hits the stable channel if we want to keep VDPAU
support.

(cherry picked from commit bf0e13a)

Note: The following might also need to be updated:
substituteStream(): WARNING: pattern '/usr/share/xcb' doesn't match anything in file 'ui/gfx/x/BUILD.gn'

(cherry picked from commit 8815c9e)

We'll need a working build of Chromium M86 for NixOS 20.03 to provide
security updates for 30 days after the release of 20.09.