Build 73.0.3683.103-1 for Ubuntu Cosmic

[riyad]

I have a working build for 73.0.3683.103-1 for ubuntu_cosmic.

Steps:

• [x] merge debian_buster into ubuntu_cosmic
• [x] use LLVM-8 to prevent clang++-7: error: unknown argument: '-fsplit-lto-unit' (see https://github.com/ungoogled-software/ungoogled-chromium-debian/commit/75f3e55921e911955805d130c96d74a510e02cd7; fixes #6)
• [x] integrate this patch to prevent base/optional.h:348:61: error: no member named 'value' in 'std::is_constructible<media::learning::Value, const base::Optional<media::learning::Value> &>' (thanks to @Eloston fixing this in https://github.com/ungoogled-software/ungoogled-chromium-debian/pull/8/commits/5beef9aca18b5976302f1bc39bf9dbcc3e543cf0)

[Eloston]

Also, I'll look to merging that last patch into debian_buster since it looks like it affects anyone that uses libstdc++

[Eloston]

Just pushed the patch: 5beef9aca18b5976302f1bc39bf9dbcc3e543cf0

[riyad]

@Eloston awesome, thank you. :+1:

I rebased the PR on top of your last commits ... there's basically nothing left besides just merging debian_buster. I'll do a rebuild with the updated set of patches. If it succeeds ...

• this PR can be merged
• you can tag the 73.0.3683.103-1.cosmic1 release
• I'll create a PR for the binary release

[riyad]

@Eloston from my side this looks good.