Fix #289, make inferred type of yield from expressions mixed

src/Psalm/Checker/Statements/ExpressionChecker.php

@@ -1961,7 +1961,8 @@ class ExpressionChecker
         }
 
         if (isset($stmt->expr->inferredType)) {
-            $stmt->inferredType = $stmt->expr->inferredType;
+            // this should be whatever the generator above returns, but *not* the return type
+            $stmt->inferredType = Type::getMixed();
         }
 
         return null;

tests/Php70Test.php

@@ -243,6 +243,7 @@ class Php70Test extends TestCase
                       return 1;
                     }
                     function foo() : Generator {
+                      /** @var int */
                       $value = yield from other_generator();
                       var_export($value);
                     }',