[Syntax] Swift libSyntax API (#11320)

* Create Swift libSyntax API

This patch is an initial implementation of the Swift libSyntax API. It
aims to provide all features of the C++ API but exposed to Swift.

It currently resides in SwiftExperimental and will likely exist in a
molten state for a while.

* Only build SwiftSyntax on macOS

utils/gyb_syntax_support/Node.py

@@ -1,6 +1,6 @@
 from __future__ import print_function
 import sys  # noqa: I201
-from kinds import SYNTAX_BASE_KINDS, kind_to_type
+from kinds import SYNTAX_BASE_KINDS, kind_to_type, lowercase_first_word
 
 
 def error(msg):
@@ -19,6 +19,7 @@ class Node(object):
     def __init__(self, name, kind=None, children=None,
                  element=None, element_name=None):
         self.syntax_kind = name
+        self.swift_syntax_kind = lowercase_first_word(name)
         self.name = kind_to_type(self.syntax_kind)
 
         self.children = children or []