dotnet/sdk needs to build for net472 during source-build

[eerhardt]

In order for OmniSharp to work with a source-built .NET SDK, we need to build our MSBuild tasks for netfx. This is because OmniSharp uses Mono. For example, see #2006.

In 5.0, we built the SDK tasks for netfx. But in dotnet/sdk#19649 we stripped the netfx TFM during the build.

@MichaelSimons @crummel @dagood @dseefeld

[MichaelSimons]

[Triage] A fix for this was submitted as a patch. The patch needs to get backported.

[lambdageek]

(Not sure if this is the same underlying issue, but I think it is)

Another symptom is that OmniSharp may fail to load some analyzers such as https://github.com/dotnet/roslyn-analyzers/blob/main/src/NetAnalyzers/Core/Microsoft.NetFramework.Analyzers/TypesShouldNotExtendCertainBaseTypes.cs
because Microsoft.CodeAnalysis.NetAnalyzers.dll from source build references System.Collections.Immutable.dll version 6.0, whereas the official SDK NetAnalyzers.dll references System.Collections.Immutable.dll version 1.2.3.0.

As a result during O# initialization Mono may print errors such as:

Failure while loading the analyzer reference 'Microsoft.CodeAnalysis.NetAnalyzers': Type Microsoft.NetFramework.Analyzers.TypesShouldNotExtendCertainBaseTypesAnalyzer has invalid vtable method slot 5 with method none

(The issue is that the type of the DiagnosticAnalyzer.SupportedDiagnostics property is ImmutableArray<DiagnosticDescriptor> where ImmutableArray<> comes from System.Collections.Immutable. Since the NetAnalyzers.dll wants a newer version of System.Collections.Immutable than what comes with OmniSharp/mono, the class initialization fails and Mono prints this unhelpful error message).

/cc @webczat

[MichaelSimons]

@lambdageek - can you please log a separate issue for the OmniSharp issue you encountered?

[MichaelSimons]

Closing this as the patch was backported - dotnet/sdk#23266

[webczat]

I think we confirmed it was exactly that issue.