* Refactor Tuple Type Syntax
This patch:
- Refactors TypeArgumentListSyntax and
TypeArgumentListSyntaxData to use the SyntaxCollection and
SyntaxCollectionData APIs.
- Refactors TupleTypeElementSyntax to own its trailing comma, and
updates the tests accordingly.
- Provides an infrastructure for promoting types to use
the SyntaxCollection APIs
* Addressed comments.
* Renamed makeBlankTypeArgumentList()
* Update makeTupleType
* Changed makeTupleType to take an element list.
* Updated comment.
* Improved API for creating TupleTypeElementListSyntax'es
* Added round-trip test
* Removed last TypeArgumentList holdovers.
* Fixed round-trip test invocation
