Update swift-syntax branch to main

Author: shahmishal

utils/update_checkout/update-checkout-config.json

@@ -95,7 +95,7 @@
                 "swiftpm": "main",
                 "swift-argument-parser": "0.3.0",
                 "swift-driver": "main",
-                "swift-syntax": "master",
+                "swift-syntax": "main",
                 "swift-stress-tester": "master",
                 "swift-corelibs-xctest": "master",
                 "swift-corelibs-foundation": "master",
@@ -118,7 +118,7 @@
                 "cmark": "swift-3.0-branch",
                 "llbuild": "swift-3.0-branch",
                 "swiftpm": "swift-3.0-branch",
-                "swift-syntax": "master",
+                "swift-syntax": "main",
                 "swift-stress-tester": "master",
                 "swift-corelibs-xctest": "swift-3.0-branch",
                 "swift-corelibs-foundation": "swift-3.0-branch",
@@ -139,7 +139,7 @@
                 "cmark": "swift-3.1-branch",
                 "llbuild": "swift-3.1-branch",
                 "swiftpm": "swift-3.1-branch",
-                "swift-syntax": "master",
+                "swift-syntax": "main",
                 "swift-stress-tester": "master",
                 "swift-corelibs-xctest": "swift-3.1-branch",
                 "swift-corelibs-foundation": "swift-3.1-branch",
@@ -160,7 +160,7 @@
                 "cmark": "swift-4.0-branch",
                 "llbuild": "swift-4.0-branch",
                 "swiftpm": "swift-4.0-branch",
-                "swift-syntax": "master",
+                "swift-syntax": "main",
                 "swift-stress-tester": "master",
                 "swift-corelibs-xctest": "swift-4.0-branch",
                 "swift-corelibs-foundation": "swift-4.0-branch",
@@ -181,7 +181,7 @@
                 "cmark": "swift-4.1-branch",
                 "llbuild": "swift-4.1-branch",
                 "swiftpm": "swift-4.1-branch",
-                "swift-syntax": "master",
+                "swift-syntax": "main",
                 "swift-stress-tester": "master",
                 "swift-corelibs-xctest": "swift-4.1-branch",
                 "swift-corelibs-foundation": "swift-4.1-branch",
@@ -318,7 +318,7 @@
                 "swiftpm": "main",
                 "swift-argument-parser": "0.3.0",
                 "swift-driver": "main",
-                "swift-syntax": "master",
+                "swift-syntax": "main",
                 "swift-stress-tester": "master",
                 "swift-corelibs-xctest": "master",
                 "swift-corelibs-foundation": "master",