Generate SYNTAX_NODES in SwiftSyntaxBuilderGeneration

[fwcd]

A small patch that generates the Swift declarations for SYNTAX_NODES (thereby also moving this declaration into its own file for #479). These are required by many gyb templates in SwiftSyntaxBuilder and let us move forward with the migration to Swift templates.

The definition of SYNTAX_NODES is analogous to the corresponding declaration in gyb_syntax_support.

cc @ahoppen @kimdv

[fwcd]

@swift-ci please test

[kimdv]

A small note on the implementation: Instead of manually concatenating EXPR_NODES, DECL_NODES etc. we use gyb to generate all of these nodes again. While this causes us to generate new instances for all of these nodes, it eliminates the redundancy of having to manually enumerate all of the different node kinds (though feel free to comment on whether you would prefer the latter approach instead).

Personally I like merge of the arrays better, otherwise we have the same node twice.
So I would prefer the

let SYNTAX_NODES: [Node] = COMMON_NODES +
    EXPR_NODES +
    DECL_NODES +
    ATTRIBUTE_NODES +
    STMT_NODES +
    GENERIC_NODES +
    TYPE_NODES +
    PATTERN_NODES +
    AVAILABILITY_NODES

[fwcd]

I don't have a strong opinion here, so if you prefer the concatenation, let's do that instead 👍

[fwcd]

@swift-ci please test

[ahoppen]

LGTM