Fix CI: name the unsigned overload type through template parameters

basic_json used number_unsigned_overload_t<basic_json> while the class was
still incomplete. MSVC rejected it as a recursive alias declaration and GCC
4.9, 5 and 7 crashed with an internal compiler error. basic_json now uses
number_unsigned_overload_type<NumberIntegerType, NumberUnsignedType>, which
also makes the GCC 5 workaround in the patch test unnecessary.

Infer reported a null dereference for the number_unsigned case in
from_json, which cannot happen for a basic_json whose number_unsigned_t is
number_integer_t; assert it.

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

tests/src/unit-same-number-types.cpp

@@ -157,10 +157,7 @@ TEST_CASE_TEMPLATE("same type for number_integer_t and number_unsigned_t", Json,
     {
         const Json source = Json::parse(R"({"a": [1, 2, 3], "b": 4})");
         const Json target = Json::parse(R"({"a": [1, 5], "c": 6})");
-        // GCC 5 crashes with an internal compiler error when instantiating patch_inplace for this type
-#if !defined(__GNUC__) || defined(__clang__) || __GNUC__ >= 6
         CHECK(source.patch(Json::diff(source, target)) == target);
-#endif
         CHECK(source.flatten().unflatten() == source);
     }