Support unloadability in DispatchProxy.

[teo-tsirpanis]

The System.Reflection.DispatchProxy type used to be incompatible with unloadability, because the AssemblyBuilder with the generated proxy types is created with AssemblyBuilderAccess.Run and because all proxy types were associated with a single one.

This PR creates a separate AssemblyBuilder, one for each AssemblyLoadContext of the base type's assembly, and passes AssemblyBuilderAccess.RunAndCollect to it (edit: only if the associated ALC is unloadable). It also cleans-up some code.

Besides allowing unneeded proxy types to be granularly unloaded, it also added support for creating proxies for the same type in different ALC. I added tests for both cases.

Fixes #60468
Fixes #62050

[d066z]

delete

[teo-tsirpanis]

The two tests I added fail on Browser because Assembly.Location returns an empty string. Any idea how to selectively disable them?

[jkotas]

I would recomend to early out from the tests if the Location is empty string. It will make the test pass in all situations when the assembly is not physically available on the disk (e.g. #43079).

[teo-tsirpanis]

Thanks for the feedback @jkotas, that was my first guess, thought there might be a more elegant way. I updated the tests.

[d066hie]

change

[teo-tsirpanis]

Hello @d066hie, would you like to explain in more detail what to change in this PR?

[buyaa-n]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[buyaa-n]

Thank you @teo-tsirpanis, LGTM

[buyaa-n]

Outerloop test failures are unrelated and reported as needed (#62305, #60753 (comment), #58616 (comment), #56165 (comment)).

Thank you for your review @jkotas @vitek-karas if you have no more comments/concerns the PR is ready for merge.

[vitek-karas]

Thanks a lot @teo-tsirpanis !!