Mark no_key_compare_map::swap noexcept

Clang-Tidy rejects a swap that is not:

  error: swap functions should be marked noexcept
         [cppcoreguidelines-noexcept-swap,performance-noexcept-swap]

It was left unmarked on the grounds that std::map::swap is only
conditionally noexcept, so an unconditional promise would be wrong for a
comparator or allocator that can throw while swapping. Both concerns are met
by taking the specification from the wrapped map rather than asserting one:
noexcept(noexcept(data.swap(other.data))). Clang-Tidy accepts that, and no
NOLINT is needed.

Last in the series of specifications that inheritance used to supply and
composition has to write out by hand.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018hxZxz8svM54c6ATEvXp5E
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-custom-object-type.cpp

@@ -162,7 +162,7 @@ class no_key_compare_map
         return data.erase(key);
     }
 
-    void swap(no_key_compare_map& other)
+    void swap(no_key_compare_map& other) noexcept(noexcept(data.swap(other.data)))
     {
         data.swap(other.data);
     }