Require parentheses around tuple in list comprehension filtering.

This affects only the tuple that appears after 'if' inside a list
comprehension. Since a truth value is expected, it's very unlikely to break
existing code. It's technically a breaking change, but it's not worth
introducing a flag for this.

RELNOTES:
  Parentheses around the tuple are now mandatory in [a for b in c if 1, 2]
PiperOrigin-RevId: 159218397
diff --git a/src/test/java/com/google/devtools/build/lib/syntax/ParserTest.java b/src/test/java/com/google/devtools/build/lib/syntax/ParserTest.java
index c28adaa..b5eedb2 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/ParserTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/ParserTest.java
@@ -689,6 +689,10 @@
     parseExpression("[x for x for y in ['a']]");
     assertContainsError("syntax error at 'for'");
     clearEvents();
+
+    parseExpression("[x for x for y in 1, 2]");
+    assertContainsError("syntax error at 'for'");
+    clearEvents();
   }
 
   @Test