This causes us to retag buffers twice.  Now only listen to semantic changes
so we only tag a buffer once.

Remove its tag-source based implementation as that is no longer necessary.

After talking to Jason, we just can't see the need for this.  We already throttle
producing tags when we receive events.  Once we've actually gone and computed tags
there's no reason to continue throttling a large amount.  We might as well just
go and report the new tags.

This allows taggers to be somewhat stateful, passing information between ProduceTags calls.

Conflicts:
	src/EditorFeatures/Core/Shared/Tagging/TagSources/ProducerPopulatedTagSource.cs

Update Using Additional Files.md

Updating readme for VS 2015 release

Update Completion List API to make it easier to provide custom CompletionItem behavior

Disable failing test

This saves us having to create many requests to the UI thread to do work.

The value of nil heap handles in EnC deltas should be zero

Make TokenOfStringCached  independent of length