Syntax: Add support for 'reasync' keyword

Author: slavapestov

test/Concurrency/Runtime/reasync.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend -emit-module-path %t/reasync.swiftmodule %S/Inputs/reasync.swift -enable-experimental-concurrency
+// RUN: %target-swift-frontend -emit-module-path %t/reasync.swiftmodule %S/Inputs/reasync.swift -enable-experimental-concurrency -verify-syntax-tree
 // RUN: %target-build-swift %s -I %t -o %t/main -module-name main
 // RUN: %target-run %t/main
 

utils/gyb_syntax_support/DeclNodes.py

@@ -76,9 +76,9 @@
     Node('FunctionSignature', kind='Syntax',
          children=[
              Child('Input', kind='ParameterClause'),
-             Child('AsyncKeyword', kind='IdentifierToken',
+             Child('AsyncOrReasyncKeyword', kind='IdentifierToken',
                    classification='Keyword',
-                   text_choices=['async'], is_optional=True),
+                   text_choices=['async', 'reasync'], is_optional=True),
              Child('ThrowsOrRethrowsKeyword', kind='Token',
                    is_optional=True,
                    token_choices=[