Add ConcurrentBag Clear APIs

[AlexRadch]

Implement ConcurrentBag.Clear API. See https://github.com/dotnet/corefx/issues/2338 issue.

[AlexRadch]

I do not clear understand Adding APIs Guidelines so test project can be compiled and tested locally in src\System.Collections.Concurrent\tests\ folder but global build return strange errors.

@Priya91 can you help me with config files to add API correctly to pass root build?

[stephentoub]

Nit: var => ThreadLocalList

[AlexRadch]

Changed

[Priya91]

@AlexRadch Can you post the build errors..

[AlexRadch]

@Priya91 msbuild.err.txt

[stephentoub]

@AlexRadch, it'll probably be easier for you if you wait for @jusinvp to finish with #13828. As part of that he's added the necessary netcoreapp11 support to the project, and once that's there it should be fairly straightforward for you to add these new methods.

[AlexRadch]

@stephentoub Ok. I am waiting when you add netcoreapp11 support to this project. Thanks.

[karelz]

@justinvp's PR is merged for 5 days now - #13828 (comment). The work should be unblocked ...

[karelz]

@AlexRadch do you plan to finish it?

[karelz]

@AlexRadch ping?

[stephentoub]

Thanks for your work on this, @AlexRadch. Since this has been sitting for a couple of months and needed to be significantly updated based on changes to the target types, I went ahead and took your commits and created a new PR at #15292.