CopyRefAssembly task should log the file copy

[KirillOsenkov]

If the CopyRefAssembly task finds that the destination .dll has the same MVID as the source, it will helpfully log a message that the copy is skipped:

roslyn/src/Compilers/Core/MSBuildTask/CopyRefAssembly.cs

Line 66 in bb11cf4

Log.LogMessageFromResources(MessageImportance.Low, "CopyRefAssembly_SkippingCopy1", DestinationPath);

However if it proceeds to copy the file over, it doesn't seem to log the message.

It would help for various analysis tools if it logged the same message as the MSBuild Copy task in case the file is copied:
"Copying ... to ..."
https://github.com/dotnet/msbuild/blob/e049bfe3dddb84270dec5fc1aaedc9664f77eb98/src/Tasks/Copy.cs#L303

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[KirillOsenkov]

Ideally also log another, diag-level message that would print the source MVID and the destination MVID in case they were different, as well as the timestamp on both source and destination. This would be incredibly valuable for incremental build investigations.

[jcouv]

Thanks for the feedback. Will take a look and tag you on PR.