cpplint.py: relax checks for "using namespace" declarations for impor…

…ting custom literals.

For example, the standard library encourages "using namespace" like:

```
using namespace std::literals::chrono_literals (since C++14)
using namespace std::literals::complex_literals (since C++14)
using namespace std::literals::string_literals (since C++14)
using namespace std::literals::string_view_literals (since C++17)
```

so those shouldn't result in lint errors.

See https://en.cppreference.com/w/cpp/symbol_index/literals

scripts/cpplint.py

@@ -280,7 +280,6 @@
     'build/include_order',
     'build/include_what_you_use',
     'build/namespaces_headers',
-    'build/namespaces_literals',
     'build/namespaces',
     'build/printf_format',
     'build/storage_class',
@@ -5352,15 +5351,10 @@ def CheckLanguage(filename, clean_lines, linenum, file_extension,
           'Did you mean "memset(%s, 0, %s)"?'
           % (match.group(1), match.group(2)))
 
-  if Search(r'\busing namespace\b', line):
-    if Search(r'\bliterals\b', line):
-      error(filename, linenum, 'build/namespaces_literals', 5,
-            'Do not use namespace using-directives.  '
-            'Use using-declarations instead.')
-    else:
-      error(filename, linenum, 'build/namespaces', 5,
-            'Do not use namespace using-directives.  '
-            'Use using-declarations instead.')
+  if Search(r'\busing namespace\b', line) and not Search(r'\b::\w+_literals\b', line):
+    error(filename, linenum, 'build/namespaces', 5,
+          'Do not use namespace using-directives.  '
+          'Use using-declarations instead.')
 
   # Detect variable-length arrays.
   match = Match(r'\s*(.+::)?(\w+) [a-z]\w*\[(.+)];', line)