minor #359 [PHP80] Prevent redefinition of PHP 8.0 classes (Firtzberg)

This PR was merged into the 1.23-dev branch.

Discussion
----------

[PHP80] Prevent redefinition of PHP 8.0 classes

Not redeclaring polyfilled classes when PHP version is 8.0 or above.
The same approach is used for the Stringable interface in [this commit](symfony/polyfill@8a3e849).

One of my scripts runs [`opcache_compile_file`](https://www.php.net/manual/en/function.opcache-compile-file.php) on vendor files, including symfony. I'm in the process of upgrading to PHP 8. Running `opcache_compile_file` on either of the modified files causes a fatal error `Fatal error: Cannot declare class ValueError/UnhandledMatchError, because the name is already in use`.
That's not the case on PHP versions below 8.

Commits
-------

f1854da Not redeclaring polyfilled classes when PHP version is target or above

Resources/stubs/UnhandledMatchError.php

@@ -1,5 +1,7 @@
 <?php
 
-class UnhandledMatchError extends Error
-{
+if (\PHP_VERSION_ID < 80000) {
+    class UnhandledMatchError extends Error
+    {
+    }
 }

Resources/stubs/ValueError.php

@@ -1,5 +1,7 @@
 <?php
 
-class ValueError extends Error
-{
+if (\PHP_VERSION_ID < 80000) {
+    class ValueError extends Error
+    {
+    }
 }