Simplify is_ordered_map to reuse has_capacity

is_ordered_map re-detected capacity() with a C++03 sizeof/vararg
trick right after has_capacity did the same detection through
is_detected. For ordered_map, the old trick took the address of
std::vector::capacity, which [namespace.std]/6 makes unspecified.
Reuse has_capacity instead, which removes the unspecified-behavior
pointer-to-std-member and two NOLINT suppressions.

Part of #5708

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-type_traits.cpp

@@ -10,10 +10,14 @@
 
 #if JSON_TEST_USING_MULTIPLE_HEADERS
     #include <nlohmann/detail/meta/type_traits.hpp>
+    #include <nlohmann/ordered_map.hpp>
 #else
     #include <nlohmann/json.hpp>
 #endif
 
+#include <map>
+#include <string>
+
 TEST_CASE("type traits")
 {
     SECTION("is_c_string")
@@ -83,4 +87,12 @@ TEST_CASE("type traits")
             }
         }
     }
+
+    SECTION("is_ordered_map")
+    {
+        using nlohmann::detail::is_ordered_map;
+
+        CHECK(is_ordered_map<nlohmann::ordered_map<std::string, int>>::value);
+        CHECK_FALSE(is_ordered_map<std::map<std::string, int>>::value);
+    }
 }