Change IBC merge to report warning, not error when IBC data are missing

[tmat]

This will allow us to choose whether to block the build or not when IBC data are missing for some assemblies (by enabling or disabling warnaserror for the warning).

[tmat]

@brianrob @jaredpar

[jaredpar]

I actually like that this as an error. Yes our build is blocked but at the same time it's blocked for a good reasons. But agree it should be configurable.

[brianrob]

By default, missing data should be an error. If you choose to try and embed data and the data is missing that should be an error. I think if you might not have data for a particular assembly and you're OK with ignoring it, then you should not call ibcmerge to begin with and should check before doing so. If we make this a warning, then its going to get abused over time and there will be assemblies out there that are inadvertently unoptimized.

Out of curiosity, what is the scenario where you want to make this a warning?

[brianrob]

I'd like to understand the scenario for this. By default I believe this should be an error - if ibcmerge is being called then no data represents an error condition.

[tmat]

By default, missing data should be an error.

Arcade treats all warnings as errors by default. But I forgot that msbuild still doesn't have warnnotaserror working :( dotnet/msbuild#3062

But agree it should be configurable.

I guess we need to check NoWarn property manually :-|

Out of curiosity, what is the scenario where you want to make this a warning?

I'm not sure if not having some assemblies optimized is an error that should block the entire build.

I'm gonna close this for now. The Roslyn build is currently blocked in master, but it seems that is just a point in time issue and we should deal with that differently.