Backward incompatible: Turn on PureOperatorBuiltins: use only enum-based built-in functions in the AST.

If this breaks your AST consumer, best is to modify it to test
against the enum values instead of doing string comparisons on
built-in function names.  This is the reason the change was made.

If you need the old behavior, you should be able to get it back by changing
PureOperatorBuiltins to be false instead of true.  This path will work for
a while, but is marked deprecated.

Also, the old behavior is tagged as release 2.4.