[SwiftSyntax] Support shorthand if-let

Author: ahoppen

test/Syntax/verify_shorthand_if_let.swift

@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift -verify-syntax-tree
+
+func foo(x: Int?) {
+  if let x {
+    print(x)
+  }
+}

utils/gyb_syntax_support/StmtNodes.py

@@ -234,7 +234,8 @@
              Child('Pattern', kind='Pattern'),
              Child('TypeAnnotation', kind='TypeAnnotation',
                    is_optional=True),
-             Child('Initializer', kind='InitializerClause'),
+             Child('Initializer', kind='InitializerClause',
+                    is_optional=True),
          ]),
 
     # unavailability-condition -> '#unavailable' '(' availability-spec ')'