Match spaces at the begining of the file

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/parser/antlr/Python3.g4

@@ -1756,8 +1756,8 @@ ASYNC : 'async';
 AWAIT : 'await';
 
 NEWLINE
- : ( // removed to speed up parsing:
-     // {atStartOfInput()}?   SPACES |
+ : (
+     {atStartOfInput()}?   SPACES |
      ( '\r'? '\n' | '\r' | '\f' ) SPACES?
    )
    {