Correctly handle using foo = std::foo inside namespaces.

[saugustine]

Summary

The gdb pretty printer misprints variables declared via
using declarations of the form:

namespace foo {
using string_view = std::string_view;

string_view bar;
}

This change fixes that, by deferring the decision to ignore
types not inside std until after desugaring.

libcxx/test/pretty_printers/gdb_pretty_printer_test.sh.cpp

void string_test() {
   ComparePrettyPrintToChars(short_string, "\"kdjflskdjf\"");
 
   std::basic_string<char, std::char_traits<char>, UncompressibleAllocator<char>>
       long_string("mehmet bizim dostumuz agzi kirik testimiz");
   ComparePrettyPrintToChars(long_string,
                             "\"mehmet bizim dostumuz agzi kirik testimiz\"");
 }
 
+namespace a_namespace {
+// To test name-lookup in the presence of using inside a namespace. Inside this
+// namespace, unqualified string_view variables will appear in the debug info as
+// "a_namespace::string_view, rather than "std::string_view".
+//
+// There is nothing special here about string_view; it's just the data structure
+// where lookup with using inside a namespace wasn't always working.
+
+using string_view = std::string_view;
+
 void string_view_test() {
   std::string_view i_am_empty;
   ComparePrettyPrintToChars(i_am_empty, "std::string_view of length 0: \"\"");
 
   std::string source_string("to be or not to be");
   std::string_view to_be(source_string);
   ComparePrettyPrintToChars(
       to_be, "std::string_view of length 18: \"to be or not to be\"");
 
   const char char_arr[] = "what a wonderful world";
   std::string_view wonderful(&char_arr[7], 9);
   ComparePrettyPrintToChars(
       wonderful, "std::string_view of length 9: \"wonderful\"");
+
+  const char char_arr1[] = "namespace_stringview";
+  string_view namespace_stringview(&char_arr1[10], 10);
+  ComparePrettyPrintToChars(
+      namespace_stringview, "std::string_view of length 10: \"stringview\"");
+}
 }
 
 void u16string_test() {
   std::u16string test0 = u"Hello World";
   ComparePrettyPrintToChars(test0, "u\"Hello World\"");
   std::u16string test1 = u"\U00010196\u20AC\u00A3\u0024";
   ComparePrettyPrintToChars(test1, "u\"\U00010196\u20AC\u00A3\u0024\"");
   std::u16string test2 = u"\u0024\u0025\u0026\u0027";
void streampos_test() {
   ComparePrettyPrintToChars(
       test1, "std::fpos with stream offset:5 with state: {count:0 value:0}");
 }
 
 int main(int argc, char* argv[]) {
   framework_self_test();
 
   string_test();
-  string_view_test();
+  a_namespace::string_view_test();
 
   u32string_test();
   tuple_test();
   unique_ptr_test();
   shared_ptr_test();
   bitset_test();
   list_test();
   deque_test();

libcxx/utils/gdb/libcxx/printers.py

def __call__(self, val):
 
         # Do not handle any type that is not a struct/class.
         if val.type.strip_typedefs().code != gdb.TYPE_CODE_STRUCT:
             return None
 
         # Don't attempt types known to be inside libstdcxx.
         typename = val.type.name or val.type.tag or str(val.type)
         match = re.match("^std::(__.*?)::", typename)
-        if match is None or match.group(1) in ["__cxx1998",
+        if match is not None and match.group(1) in ["__cxx1998",
                                                     "__debug",
                                                     "__7",
                                                     "__g"]:
             return None
 
         # Handle any using declarations or other typedefs.
         typename = _prettify_typename(val.type)
         if not typename:
             return None
         without_generics = _remove_generics(typename)
         lookup_name = _remove_std_prefix(without_generics)