[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/ExprNodes.py

@@ -65,7 +65,7 @@ EXPR_NODES = [
     # !true
     Node('PrefixOperatorExpr', kind='Expr',
          children=[
-             Child('Operator', kind='PrefixOperatorToken',
+             Child('OperatorToken', kind='PrefixOperatorToken',
                    is_optional=True),
              Child('PostfixExpression', kind='Expr'),
          ]),
@@ -111,4 +111,9 @@ EXPR_NODES = [
                    is_optional=True),
              Child('Digits', kind='IntegerLiteralToken'),
          ]),
+
+    Node('StringLiteralExpr', kind='Expr',
+         children=[
+             Child("StringLiteral", kind='StringLiteralToken')
+         ])
 ]