contrib: filter out a new Clang warning

Filter out:
libcpp/lex.cc:1289:7: warning: use of the 'likely' attribute is a C++20 extension [-Wc++20-attribute-extensions]

contrib/ChangeLog:

	* filter-clang-warnings.py: Filter out
	-Wc++20-attribute-extensions in lex.cc.

contrib/filter-clang-warnings.py

@@ -53,7 +53,8 @@ def skip_warning(filename, message):
             'genautomata.cc': ['-Wstring-plus-int'],
             'fold-const-call.cc': ['-Wreturn-type'],
             'gfortran.texi': [''],
-            'libtool': ['']
+            'libtool': [''],
+            'lex.cc': ['-Wc++20-attribute-extensions'],
     }
 
     for name, ignores in ignores.items():