Do not allow code with warnings to pass CI

[npaun]

This PR enables -Werror, when building in the CI. As a result, warnings are treated as a fatal error. This ensures that warnings will be addressed, and won't pile up over time in our codebase. Here are some specific details about the configuration.

• Warnings aren't fatal locally. This is for convenience while working on code.
• Warnings aren't fatal in external/. This is because these warnings come from our dependencies, and need to be fixed there.
• Deprecation warnings and #warning are not fatal. They're just here to give us a heads up that something will need to be fixed eventually.
• Warnings are only fatal on Linux builds. I didn't want to make this a maddening quest of fixing macOS, Windows AND Linux compiler warnings every time we do a PR.

[anonrig]

Can you also update compile_flags.txt?

[danlapid]

Can you also update compile_flags.txt?

I think that would break "Warnings aren't fatal locally. This is for convenience while working on code."
As it will cause clangd to show error

[fhanau]

• Warnings aren't fatal locally. This is for convenience while working on code.
• Warnings are only fatal on Linux builds. I didn't want to make this a maddening quest of fixing macOS, Windows AND Linux compiler warnings every time we do a PR.

I think these two aspects are key, thank you for covering them. 1) is important for development velocity, 2) means we won't have to fix the many unhelpful Windows warnings we have. Including macOS via build:unix could be a good option as both systems share the vast majority of warnings when they happen but it conceivably could make things harder for non-mac developers and is not needed at this stage.

[fhanau]

✅