[ruby/prism] Raise `void value expression` in begin clausesHEADmaster

In some cases Prism was either not raising an appropriate `void value
expression` error, or raising that error when the syntax is considered
valid.

To fix this Prism needs to check whether we have other clauses on the
`begin` rather than just returning `cast->statements`.

* If the `cast->statements` are null and the `cast->ensure_clause` is
not null, set the code to `cast->ensure_clause`
* else
  * If there is a `cast->rescue_clause`
    * Check if `cast->statements` are null and `cast->rescue_clause->statements`
    are null, and return `NULL`
    * Check if there is an `else_clause`, and set the node to
      `cast->else_clause`.
    * Otherwise return `cast->statements` as the node
  * return `cast->statements` as the node

See tests for test cases. Note I took these directly from CRuby so if
desired I can delete them since the test will now pass. This only fixes
one test in the `test_parse` file, taking failures from 14 to 13.

This fixes `TestParse#test_void_value_in_rhs` and is related to
issue #2791.

https://github.com/ruby/prism/commit/398152b412

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