Detect correctly multitargeted apps for trim/AOT/single-file warnings (…

…#35767)


The warnings introduced by
 #34077 and described in
 https://learn.microsoft.com/en-us/dotnet/core/compatibility/sdk/8.0/trimming-unsupported-targetframework
 are unnecessary noise for projects that multitarget to include a
 TargetFramework that is compatible with
 trimming/AOT/single-file, and is a low enough version to ensure
 that it will be picked over any other TFM's assets when the
 library is consumed in a trimmed/AOT'd/single-file app.

This fixes the issue by detecting correctly multi-targeted
libraries and suppressing the warnings in these cases.

Note that prior to the .NET 8 SDK, netstandard libraries would
still get the `IsTrimmable` attribute embedded in the assembly
when setting `IsTrimmable`, and could use trim analysis despite
incomplete warnings due to unannotated ref assemblies. With the
.NET 8 SDK this is no longer the case. Even for correctly
multi-targeted libraries, the netstandard output assembly will no
longer contain the `IsTrimmable` attribute. We debated whether it
was worth warning in this case, but decided that the negative
impact of the warning on library developers following the "golden
path" was too large to justify keeping this as a warning.

This solution addresses a correctness problem that we wanted to
avoid (the correctness problem: allowing an inadvertently
non-"trimmable" netstandard asset to be consumed in a trimmed app
that uses a supported TFM).

It's still possible to get into that situation in an app that
explicitly references the netstandard assembly, or that targets
an EOL TFM (causing it to consume the netstandard asset from a
library that multitargets `netstandard2.0;net6.0` for example).

Fixes #35528

src/Tasks/Common/Resources/Strings.resx

@@ -931,17 +931,17 @@ You may need to build the project on another operating system or architecture, o
   </data>
   <data name="IsAotCompatibleUnsupported" xml:space="preserve">
     <value>NETSDK1210: IsAotCompatible and EnableAotAnalyzer are not supported for the target framework. Consider multi-targeting to a supported framework to enable ahead-of-time compilation analysis, and set IsAotCompatible only for the supported frameworks. For example:
-&lt;IsAotCompatible Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net7.0'))"&gt;true&lt;/IsAotCompatible&gt;</value>
+&lt;IsAotCompatible Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', '{0}'))"&gt;true&lt;/IsAotCompatible&gt;</value>
     <comment>{StrBegin="NETSDK1210: "}</comment>
   </data>
   <data name="EnableSingleFileAnalyzerUnsupported" xml:space="preserve">
     <value>NETSDK1211: EnableSingleFileAnalyzer is not supported for the target framework. Consider multi-targeting to a supported framework to enable single-file analysis, and set EnableSingleFileAnalyzer only for the supported frameworks. For example:
-&lt;EnableSingleFileAnalyzer Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))"&gt;true&lt;/EnableSingleFileAnalyzer&gt;</value>
+&lt;EnableSingleFileAnalyzer Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', '{0}'))"&gt;true&lt;/EnableSingleFileAnalyzer&gt;</value>
     <comment>{StrBegin="NETSDK1211: "}</comment>
   </data>
   <data name="IsTrimmableUnsupported" xml:space="preserve">
     <value>NETSDK1212: IsTrimmable and EnableTrimAnalyzer are not supported for the target framework. Consider multi-targeting to a supported framework to enable trimming, and set IsTrimmable only for the supported frameworks. For example:
-&lt;IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))"&gt;true&lt;/IsTrimmable&gt;</value>
+&lt;IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', '{0}'))"&gt;true&lt;/IsTrimmable&gt;</value>
     <comment>{StrBegin="NETSDK1212: "}</comment>
   </data>
   <data name="Net8NotCompatibleWithDev177" xml:space="preserve">