Emit '[Embedded]' attribute for all polyfills

[Sergio0694]

Closes #50, #103

Description

This PR updates the generator to also emit the [Embedded] attribute for polyfills, to avoid conflicts for internal types.

[cremor]

Would this work if you don't have the latest SDK or VS installed? Because according to this you couldn't declare the EmbeddedAttribute yourself if the compiler would have created one prior to VS 17.13. See also dotnet/roslyn#76523

Also, just to have it documented here too: @MarkKharitonov found a problem with this solution, see #50 (comment)

[Sergio0694]

@cremor yeah this will work fine, I'm still using Roslyn 4.3.0, and not that new API that was added 🙂

[cremor]

I didn't mean the new API. I meant that quote from the PR dotnet/roslyn#76523:

Today, [...] if we need to generate it, we error if the user defines one.

I understand this as:

If you use a compiler version before VS 17.13
and
you define EmbeddedAttribute yourself (e.g. by using PolySharp with this new changes)
and
you have something in your code that would lead Roslyn to emit the EmbeddedAttribute itself

then you would get an error.

I haven't tested this. I just wanted to raise awareness.

[jviau]

Small suggestion. Recent Roslyn APIs have an convenience API .AddEmbeddedAttributeDefinition(), which will emit the EmbeddedAttribute type for you. As opposed to including it directly like you are in this PR. If you make including of the [Embedded] marker on all the types opt-in via an MSBuild variable, then you can also opt in having the EmbeddedAttribute at all via conditionally calling that API.

You could re-create this helper if you are not on the newer Roslyn packages: https://github.com/dotnet/roslyn/blob/51d2ed9e47364b57024f7715c2e2b2e385fe1454/src/Compilers/Core/Portable/SourceGeneration/IncrementalContexts.cs#L144