Add ManualResetValueTaskSourceCore / AsyncIterateMethodBuilder to Microsoft.Bcl.Async...

Add ManualResetValueTaskSourceCore / AsyncIterateMethodBuilder to Microsoft.Bcl.AsyncInterfaces (dotnet/corefx#37320)

* Add ManualResetValueTaskSourceCore / AsyncIterateMethodBuilder to Microsoft.Bcl.AsyncInterfaces

These two types needed modifications to target .NET Standard 2.0 and are the necessary pieces to enable the compiler to compile async iterators.

- Copied ManualResetValueTaskSourceCore.cs from coreclr and tweaked it.  I opted to do this rather than ifdef because the changes are not localized and I didn't want to significantly perturb the primary implementation.
- Added a few ifdefs to the shared AsyncIteratorMethodBuilder.  It already had ifdefs, so I just added to it.
- Added a test project, and included the existing ManualResetValueTaskSourceCore tests.  I had to disable two of the tests because of some of the optimization differences.
- Augmented those tests to validate that the compiler is able to successfully generate iterators and await foreach them.

* Address PR feedback

Commit migrated from https://github.com/dotnet/corefx/commit/07b6760a1a84251a50f744e585e4da5f71a25e68