Add FreestandingMacroSyntax trait for freestanding macros.

Author: DougGregor

gyb_syntax_support/DeclNodes.py

@@ -924,6 +924,7 @@
     # e.g., "#embed("filename.txt")"
     Node('MacroExpansionDecl',
          name_for_diagnostics="pound literal declaration", kind='Decl',
+         traits=['FreestandingMacroExpansion'],
          children=[
              Child('PoundToken', kind='PoundToken',
                    description='The `#` sign.'),

gyb_syntax_support/ExprNodes.py

@@ -653,6 +653,7 @@
     # e.g., "#embed("filename.txt")"
     Node('MacroExpansionExpr',
          name_for_diagnostics="pound literal expression", kind='Expr',
+         traits=['FreestandingMacroExpansion'],
          children=[
              Child('PoundToken', kind='PoundToken',
                    description='The `#` sign.'),

gyb_syntax_support/Traits.py

@@ -43,6 +43,26 @@ def __init__(self, trait_name, description=None, children=None):
               Child('RightParen', kind='RightParenToken'),
           ]),
 
+    Trait('FreestandingMacroExpansion',
+          children=[
+             Child('PoundToken', kind='PoundToken'),
+             Child('Macro', kind='IdentifierToken'),
+             Child('GenericArguments', kind='GenericArgumentClause',
+                   is_optional=True),
+             Child('LeftParen', kind='LeftParenToken',
+                   is_optional=True),
+             Child('ArgumentList', kind='TupleExprElementList',
+                   collection_element_name='Argument'),
+             Child('RightParen', kind='RightParenToken',
+                   is_optional=True),
+             Child('TrailingClosure', kind='ClosureExpr',
+                   is_optional=True),
+             Child('AdditionalTrailingClosures',
+                   kind='MultipleTrailingClosureElementList',
+                   collection_element_name='AdditionalTrailingClosure',
+                   is_optional=True),
+          ]),
+
     Trait('WithTrailingComma',
           children=[
               Child('TrailingComma', kind='CommaToken', is_optional=True),