Fix clang-tidy and clang 6 in the #5066 regression test

ci_clang_tidy asked for emplace_back instead of push_back. The
push_back is the point of the test: #5066 is about the implicit
conversion from json to the vector's value type, which emplace_back
would bypass. Silence the check on that line.

clang 5 and 6 cannot instantiate std::variant<json> from libstdc++ 10's
<variant> ("cannot cast private base class"), which broke
ci_test_compilers_clang (6). Tested with the CI images: clang 7 to 11
compile and pass, including clang 11 with libstdc++ 10. Skip the runtime
check for clang before 7; the static_assert still runs.

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

tests/src/unit-regression2.cpp

@@ -775,9 +775,14 @@ TEST_CASE("regression tests 2")
         static_assert(!nlohmann::detail::is_detected<nlohmann::detail::get_template_function, const json&, std::variant<json>>::value,
                       "std::variant<json> must not be retrievable via get<>()");
 
+        // clang before 7 cannot instantiate libstdc++'s std::variant<json>
+#if !(defined(__clang__) && __clang_major__ < 7)
+        // push_back, not emplace_back: #5066 needs the implicit conversion
+        // from json to the vector's value type
         std::vector<std::variant<json>> v;
-        v.push_back(json(1));
+        v.push_back(json(1)); // NOLINT(hicpp-use-emplace,modernize-use-emplace)
         CHECK(std::get<0>(v[0]) == 1);
+#endif
     }
 #endif
 }