GDScript: Adjust STATIC_CALLED_ON_INSTANCE warning to not default to the native type, and to not trigger on self-calls

Not defaulting to the native type rationale:

Defaulting to the native type is less than useful, as:

* There are very few native types that are extensible and have static methods.
* Defaulting to the native type does not account for a method being script-defined.

While the "real fix" would be to carefully track the source of the method, the get_function_signature method is already complicated enough.

This will at least ensure the resulting code should always be valid.

Not triggering on self-calls rationale:

Found in PR comment https://github.com/godotengine/godot/pull/85918#issuecomment-1935864459

```
static func example():
	pass

func example2():
	example() # self-call on static function
```

Disabling this warning on self-calls is:

* Consistent with other languages
* Important for anonymous classes (where the output code is unusable)

modules/gdscript/gdscript_analyzer.cpp

@@ -3360,12 +3360,8 @@ void GDScriptAnalyzer::reduce_call(GDScriptParser::CallNode *p_call, bool p_is_a
 			parser->push_warning(p_call, GDScriptWarning::RETURN_VALUE_DISCARDED, p_call->function_name);
 		}
 
-		if (method_flags.has_flag(METHOD_FLAG_STATIC) && !is_constructor && !base_type.is_meta_type && !(is_self && static_context)) {
-			String caller_type = String(base_type.native_type);
-
-			if (caller_type.is_empty()) {
-				caller_type = base_type.to_string();
-			}
+		if (method_flags.has_flag(METHOD_FLAG_STATIC) && !is_constructor && !base_type.is_meta_type && !is_self) {
+			String caller_type = base_type.to_string();
 
 			parser->push_warning(p_call, GDScriptWarning::STATIC_CALLED_ON_INSTANCE, p_call->function_name, caller_type);
 		}

modules/gdscript/tests/scripts/parser/warnings/static_called_on_instance.gd

@@ -1,11 +1,23 @@
-class Player:
-	var x = 3
+class_name TestStaticCalledOnInstance
+
+class Inner:
+	static func static_func():
+		pass
+
+static func static_func():
+	pass
 
 func test():
-	# These should not emit a warning.
-	var _player = Player.new()
-	print(String.num_uint64(8589934592)) # 2 ^ 33
+	print(String.num_uint64(8589934592))
+	var some_string := String()
+	print(some_string.num_uint64(8589934592)) # Warning.
+
+	TestStaticCalledOnInstance.static_func()
+	static_func()
+	self.static_func()
+	var other := TestStaticCalledOnInstance.new()
+	other.static_func() # Warning.
 
-	# This should emit a warning.
-	var some_string = String()
-	print(some_string.num_uint64(8589934592)) # 2 ^ 33
+	Inner.static_func()
+	var inner := Inner.new()
+	inner.static_func() # Warning.

modules/gdscript/tests/scripts/parser/warnings/static_called_on_instance.out

@@ -1,7 +1,15 @@
 GDTEST_OK
 >> WARNING
->> Line: 11
+>> Line: 13
 >> STATIC_CALLED_ON_INSTANCE
 >> The function "num_uint64()" is a static function but was called from an instance. Instead, it should be directly called from the type: "String.num_uint64()".
+>> WARNING
+>> Line: 19
+>> STATIC_CALLED_ON_INSTANCE
+>> The function "static_func()" is a static function but was called from an instance. Instead, it should be directly called from the type: "TestStaticCalledOnInstance.static_func()".
+>> WARNING
+>> Line: 23
+>> STATIC_CALLED_ON_INSTANCE
+>> The function "static_func()" is a static function but was called from an instance. Instead, it should be directly called from the type: "Inner.static_func()".
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