Enable building net472 (FullFrameworkTFM) during source-build

[dagood]

In source-build 5.0, there's a patch to remove the middle line here:

msbuild/src/Directory.Build.props

Lines 29 to 31 in aac64bb

	<LibraryTargetFrameworks>$(FullFrameworkTFM);netstandard2.0</LibraryTargetFrameworks>
	<LibraryTargetFrameworks Condition="'$(DotNetBuildFromSource)' == 'true'">netstandard2.0</LibraryTargetFrameworks>
	<LibraryTargetFrameworks Condition="'$(MonoBuild)'=='true'">$(FullFrameworkTFM)</LibraryTargetFrameworks>

We need to build net472 in 6.0, too, so we have this patch:

https://github.com/dotnet/installer/blob/9326af28148706e3e89ef2d61ebb6fc1adde21d9/src/SourceBuild/tarball/patches/msbuild/0003-Restore-building-all-TFMs-for-source-build.patch
(Added in dotnet/installer#12472)

This is needed for downstream repos to target net472 in source-build so Omnisharp will work:

• dotnet/sdk needs to build for net472 during source-build source-build#2548
• Omnisharp is broken in 6.0 source-build#2478

I looked into the history a bit and it seems that this patch wasn't included in 6.0 ArPow onboarding simply because of a timing issue, not for any larger reason:

• MSBuild 6.0 ArPow onboarding (Onboarding to ArPow (arcade-powered source-build) #6387) was submitted 2021-04-29
• The 5.0 patch to add net472 (Omnisharp Razor DLL fix source-build#2178) was merged 2021-06-14

/cc @dotnet/source-build-internal @rainersigwald @eerhardt

[rainersigwald]

No need to leave it as it is. I believe it's a vestige of earlier days when we didn't have the ReferenceAssemblies packages and it was hard to build for Framework on Core on non-Windows. But we're in the future now.

[eerhardt]

NOTE: we will also need to tweak

msbuild/src/Build/Microsoft.Build.csproj

Lines 7 to 9 in 808b2ae

	<TargetFrameworks>net6.0</TargetFrameworks>
	<TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(FullFrameworkTFM);net6.0</TargetFrameworks>
	<TargetFrameworks Condition="'$(MonoBuild)'=='true'">$(RuntimeOutputTargetFrameworks)</TargetFrameworks>

Because it doesn't use $(LibraryTargetFrameworks). I believe we can just make those 3 lines:

 <TargetFrameworks>$(FullFrameworkTFM);net6.0</TargetFrameworks> 
 <TargetFrameworks Condition="'$(MonoBuild)'=='true'">$(RuntimeOutputTargetFrameworks)</TargetFrameworks> 

[MichaelSimons]

This isssue can be closed. Source-build now excludes all Full Fx TFMs.