string_view: Add defaulted copy constructor and assignment

The defaulted copy constructor and assignment were removed to work around
a gdb bug, but gdb now works without the workaround.

PiperOrigin-RevId: 706694737
Change-Id: I5da06a29094f28a45a5910eab1f7a5155b34d57f

absl/strings/string_view.h

@@ -204,9 +204,8 @@ class ABSL_ATTRIBUTE_VIEW string_view {
   constexpr string_view(absl::Nullable<const char*> data, size_type len)
       : ptr_(data), length_(CheckLengthInternal(len)) {}
 
-  // NOTE: Harmlessly omitted to work around gdb bug.
-  //   constexpr string_view(const string_view&) noexcept = default;
-  //   string_view& operator=(const string_view&) noexcept = default;
+  constexpr string_view(const string_view&) noexcept = default;
+  string_view& operator=(const string_view&) noexcept = default;
 
   // Iterators