Make sure output start filename is not freed early

As filenames are no longer interned, we need to keep a reference
to the zend_string to make sure it isn't freed.

To avoid a no

Make sure output start filename is not freed early

As filenames are no longer interned, we need to keep a reference
to the zend_string to make sure it isn't freed.

To avoid a nominal source compatibility break, create a new member
in the globals.

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