Declare no_key_compare_map's accessors noexcept

The GCC C++20 job builds with -Wnoexcept and -Werror, and the standard
library takes noexcept(c.begin()) and noexcept(c.end()) in ranges_base.h and
range_access.h. Forwarding to std::map without repeating its noexcept made
those expressions false, which the warning reports as an error:

  error: noexcept-expression evaluates to 'false' because of a call to
         no_key_compare_map<...>::begin()          [-Werror=noexcept]
  note:  but ... does not throw; perhaps it should be declared 'noexcept'

Give the accessors the exception specification of what they forward to.
std::map declares begin, end, cbegin, cend, empty, size, max_size, and clear
noexcept, so the wrapper does too. swap is left alone: std::map's is only
conditionally noexcept, and nothing asks for it.

void_erase_map is unaffected because it still derives from std::map and
inherits accessors that already carry the specification.

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

@@ -59,44 +59,44 @@ class no_key_compare_map
     template<class InputIt>
     no_key_compare_map(InputIt first, InputIt last) : data(first, last) {}
 
-    iterator begin()
+    iterator begin() noexcept
     {
         return data.begin();
     }
-    iterator end()
+    iterator end() noexcept
     {
         return data.end();
     }
-    const_iterator begin() const
+    const_iterator begin() const noexcept
     {
         return data.begin();
     }
-    const_iterator end() const
+    const_iterator end() const noexcept
     {
         return data.end();
     }
-    const_iterator cbegin() const
+    const_iterator cbegin() const noexcept
     {
         return data.cbegin();
     }
-    const_iterator cend() const
+    const_iterator cend() const noexcept
     {
         return data.cend();
     }
 
-    bool empty() const
+    bool empty() const noexcept
     {
         return data.empty();
     }
-    size_type size() const
+    size_type size() const noexcept
     {
         return data.size();
     }
-    size_type max_size() const
+    size_type max_size() const noexcept
     {
         return data.max_size();
     }
-    void clear()
+    void clear() noexcept
     {
         data.clear();
     }