Add ETW enabled checks to ConcurrentBag implementation (case 1230447)

This change modifies the `ConcurrentBag` implementation from corefx.
Since we cannot update the corefx submodule, make a copy of the
ConcurrentBag.cs file, fix the issue, and point the build at this copy.

The change is in the `TrySteal` method. It makes calls on the
`CDSCollectionETWBCLProvider`, which is removed by the manged linker
by default. Those calls should be wrapped in a
`CDSCollectionETWBCLProvider.Log.IsEnabled()` if block, so the linker
can understand to not make the calls.

mcs/class/System/common.sources

@@ -906,7 +906,7 @@ corefx/SR.cs
 ../../../external/corefx/src/System.Collections/src/System/Collections/Generic/StackDebugView.cs
 
 ../../../external/corefx/src/System.Collections.Concurrent/src/System/Collections/Concurrent/BlockingCollection.cs
-../../../external/corefx/src/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentBag.cs
+../../../external/corefx-bugfix/src/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentBag.cs
 ../../../external/corefx/src/System.Collections.Concurrent/src/System/Collections/Concurrent/CDSCollectionETWBCLProvider.cs
 
 ../../../external/corefx/src/System.IO.Compression/src/System/IO/Compression/CompressionLevel.cs