[AppHost] Make MSBuild trim-friendly

[kasperk81]

#13175 is merged. A next possible improvement could be making MSBuild trim friendly:

--- a/src/MSBuild/MSBuild.csproj
+++ b/src/MSBuild/MSBuild.csproj
@@ -15,6 +15,8 @@
          versions of this project -->
     <RuntimeIdentifiers Condition="'$(DotNetBuildSourceOnly)' != 'true'">win7-x86;win7-x64</RuntimeIdentifiers>
     <UseRidGraph>true</UseRidGraph>
+    <PublishTrimmed>true</PublishTrimmed>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>

     <EnableDefaultItems>false</EnableDefaultItems>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>

Repro:

$ .dotnet/dotnet publish -f net10.0 src/MSBuild/MSBuild.csproj
...
Build failed with 29 error(s) in 2.6s

Two concrete benefits of making MSBuild trim-friendly:

• Code bloat reduction in the final executable
• Paves the path for AOT compatibility (~83% of PublishAot errors are trimming-related)

Not pitching AOT as a main goal here, but making the codebase trimming-safe naturally moves things in that direction if/when that ever becomes interesting.

[baronfel]

I view this as mostly a dupe of #12141 - we're actively looking into this space, but trimming/AOT is only realistically even an option for evaluation - execution will always require loading arbitrary unknown libraries. The bigger set of questions are around how AOT clients should interact with the backing MSBuild engine, and we'll be tackling that in the scope of #12141.