Raise void value expression in begin

[eileencodes]

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.