`llvm::ArrayRef<T>` does not define `erase`. However, since the intent
here is to remove the last n elements, we can use `drop_back` instead.
Furthermore, replace the direct use of `Layout` with `getLayout()`.
This was identified by gcc 8.2.
* Generate libSyntax API
This patch removes the hand-rolled libSyntax API and replaces it with an
API that's entirely automatically generated. This means the API is
guaranteed to be internally stylistically and functionally consistent.
Previously, users of TokenSyntax would always deal with RC<TokenSyntax>
which is a subclass of RawSyntax. Instead, provide TokenSyntax as a
fully-realized Syntax node, that will always exist as a leaf in the
Syntax tree.
This hides the implementation detail of RawSyntax and SyntaxData
completely from clients of libSyntax, and paves the way for future
generation of Syntax nodes.
