Generic/DuplicateClassName: performance fix

As things were, the sniff would unconditionally walk a complete file, making it one of the top 15 slowest sniffs.

However, OO structures in PHP cannot be nested, so once we've found an OO declaration, we can skip to the end of it before continuing the token walking. See: https://3v4l.org/pbSTG

This small tweak makes a significant difference in the sniff performance without any impact on the sniff results.

src/Standards/Generic/Sniffs/Classes/DuplicateClassNameSniff.php

@@ -109,6 +109,10 @@ public function process(File $phpcsFile, $stackPtr)
                         ];
                     }
                 }//end if
+
+                if (isset($tokens[$stackPtr]['scope_closer']) === true) {
+                    $stackPtr = $tokens[$stackPtr]['scope_closer'];
+                }
             }//end if
 
             $stackPtr = $phpcsFile->findNext($findTokens, ($stackPtr + 1));