Raise void value expression in begin clauses

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.

src/prism.c

@@ -1183,6 +1183,31 @@ pm_check_value_expression(pm_node_t *node) {
                 return NULL;
             case PM_BEGIN_NODE: {
                 pm_begin_node_t *cast = (pm_begin_node_t *) node;
+
+                if (cast->statements == NULL && cast->ensure_clause != NULL) {
+                    node = (pm_node_t *) cast->ensure_clause;
+                }
+                else {
+                    if (cast->rescue_clause != NULL) {
+                        if (cast->rescue_clause->statements == NULL) {
+                            return NULL;
+                        }
+                        else if (cast->else_clause != NULL) {
+                            node = (pm_node_t *) cast->else_clause;
+                        }
+                        else {
+                            node = (pm_node_t *) cast->statements;
+                        }
+                    }
+                    else {
+                        node = (pm_node_t *) cast->statements;
+                    }
+                }
+
+                break;
+            }
+            case PM_ENSURE_NODE: {
+                pm_ensure_node_t *cast = (pm_ensure_node_t *) node;
                 node = (pm_node_t *) cast->statements;
                 break;
             }

test/prism/errors_test.rb

@@ -1630,6 +1630,41 @@ class << (return)
       ]
     end
 
+    def test_void_value_expression_in_begin_statement
+      source = <<~RUBY
+        x = return 1
+        x = return, 1
+        x = 1, return
+        x, y = return
+        x = begin return ensure end
+        x = begin ensure return end
+        x = begin return ensure return end
+        x = begin return; rescue; return end
+        x = begin return; rescue; return; else return end
+        x = begin; return; rescue; retry; end
+      RUBY
+
+      message = 'unexpected void value expression'
+      assert_errors expression(source), source, [
+        [message, 4..12],
+        [message, 17..23],
+        [message, 34..40],
+        [message, 48..54],
+        [message, 65..71],
+        [message, 100..106],
+        [message, 121..127],
+        [message, 156..162],
+        [message, 222..228],
+        [message, 244..250],
+      ]
+
+      refute_error_messages("x = begin return; rescue; end")
+      refute_error_messages("x = begin return; rescue; return; else end")
+      refute_error_messages("x = begin; rescue; retry; end")
+      refute_error_messages("x = begin 1; rescue; retry; ensure; end")
+      refute_error_messages("x = begin 1; rescue; return; end")
+    end
+
     def test_void_value_expression_in_def
       source = <<~RUBY
         def (return).x