Split the generated SyntaxNodes.swift file into multiple files

This way, the files go down to a manageable file size again that can
be view using Xcode.

Author: ahoppen

Sources/SwiftSyntax/SyntaxNodesBase.swift.gyb renamed to Sources/SwiftSyntax/SyntaxBaseNodes.swift.gyb

@@ -21,18 +21,11 @@
 
 %{
 """
-Each Syntax node implements the protocol of a more generic node. For example,
-StructDeclSyntax implements DeclSyntax and can be used in contexts
-where DeclSyntax is expected.
-
-Each node will have:
-- An accessor for each child that will lazily instantiate it.
-- A `withX(_ x: XSyntax)` method for each child that will return a new Syntax
-  node with the existing X child replaced with the passed-in version. This is a
-  way to incrementally transform nodes in the tree.
-- An `addX(_ x: XSyntax)` method for children that are collections. This will
-  append the provided node to the collection and return a new node with that
-  collection replaced.
+This gyb-file generates the syntax nodes for SwiftSyntax. To keep the generated
+files at a managable file size, it is to be invoked multiple times with the
+global variable EMIT_KIND set to a base kind listed in
+gyb_syntax_support/kinds.py:SYNTAX_BASE_KINDES. It then only emits those syntax
+nodes whose base kind are that specified kind.
 """
 }%
 
@@ -44,6 +37,8 @@ Each node will have:
 %   elif node.collection_element:
 %     # Handled in SyntaxCollections.swift.gyb
 %     pass
+%   elif node.base_kind != EMIT_KIND:
+%     pass
 %   else:
 %     # We are actually handling this node now
 // MARK: - ${node.name}
@@ -84,7 +79,6 @@ public struct ${node.name}: ${base_type}Protocol {
     assert(data.raw.kind == .${node.swift_syntax_kind})
     self._syntaxNode = Syntax(data)
   }
-
 %     for child in node.children:
 %       child_node = NODE_MAP.get(child.syntax_kind)
 %