Make compatible with PHP < 7.4

Author: thekid

src/main/php/lang/ast/emit/PHP.class.php

@@ -387,7 +387,7 @@ protected function emitEnum($result, $enum) {
       }');
       $result->out->write('public static function from($value) {
         if ($r= self::$values[$value] ?? null) return $r;
-        throw new \ValueError(\util\Objects::stringOf($value)." is not a valid backing value for enum \"".self::class."\"");
+        throw new \Exception(\util\Objects::stringOf($value)." is not a valid backing value for enum \"".self::class."\"");
       }');
     } else {
       $result->out->write('public $name;');

src/test/php/lang/ast/unittest/emit/EnumTest.class.php

@@ -89,7 +89,7 @@ public static function run() {
         try {
           self::from("illegal");
           throw new IllegalStateException("No exception raised");
-        } catch (\ValueError $expected) {
+        } catch (\Exception $expected) {
           return $expected->getMessage();
         }
       }