[libc++] Add default copy ctor to "__chrono/exception.h"

After PR#90394, "__chrono/exception.h" will trigger
"deprecated-copy-with-user-provided-dtor" warning on Windows x64
runtime testing with ToT Clang. This patch addresses this issue by
explicitly adding those default copy ctors.

Author: zeroomega

libcxx/include/__chrono/exception.h

@@ -48,6 +48,9 @@ class nonexistent_local_time : public runtime_error {
                             "creating an nonexistent_local_time from a local_info that is not non-existent");
   }
 
+  nonexistent_local_time(const nonexistent_local_time&) = default;
+  nonexistent_local_time& operator=(const nonexistent_local_time&) = default;
+
   _LIBCPP_AVAILABILITY_TZDB _LIBCPP_EXPORTED_FROM_ABI ~nonexistent_local_time() override; // exported as key function
 
 private:
@@ -89,6 +92,9 @@ class ambiguous_local_time : public runtime_error {
                             "creating an ambiguous_local_time from a local_info that is not ambiguous");
   }
 
+  ambiguous_local_time(const ambiguous_local_time&) = default;
+  ambiguous_local_time& operator=(const ambiguous_local_time&) = default;
+
   _LIBCPP_AVAILABILITY_TZDB _LIBCPP_EXPORTED_FROM_ABI ~ambiguous_local_time() override; // exported as key function
 
 private: