Preserve `pass` statements in the Skylark AST.

RELNOTES: none
PiperOrigin-RevId: 173125138
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Statement.java b/src/main/java/com/google/devtools/build/lib/syntax/Statement.java
index 1f08bbb..43363fc 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/Statement.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Statement.java
@@ -33,6 +33,7 @@
     FUNCTION_DEF,
     IF,
     LOAD,
+    PASS,
     RETURN,
   }