CopyOnWrite: make MSB3021 suppressable

[KirillOsenkov]

MSBuild has recently made a fix to make MSB3021 suppressible:
dotnet/msbuild#9217

CopyOnWrite SDK needs to port this fix. Otherwise we don't have compatibility with the classic task. Switching to CopyOnWrite will break the build because the warning is no longer suppressible.

[KirillOsenkov]

This is where the error code is missing:

MSBuildSdks/src/CopyOnWrite/Copy.cs

Line 801 in c80b034

LogDiagnostic("Got {0} copying {1} to {2} and HR is {3}", e.ToString(),

@erikmav @inthemedium

Without the error code, we can't suppress the warning.