GDScript: Fix temp values being written without proper clear

Temporary values in the stack were not being properly cleared when the
return value of calls were discarded, which can cause memory issues
especially for reference types like PackedByteArray.

modules/gdscript/tests/scripts/runtime/features/does_not_override_temp_values.gd

@@ -0,0 +1,17 @@
+# https://github.com/godotengine/godot/issues/71177
+
+func test():
+	builtin_method()
+	builtin_method_static()
+	print("done")
+
+func builtin_method():
+	var pba := PackedByteArray()
+	@warning_ignore(return_value_discarded)
+	pba.resize(1) # Built-in validated.
+
+
+func builtin_method_static():
+	var _pba := PackedByteArray()
+	@warning_ignore(return_value_discarded)
+	Vector2.from_angle(PI) # Static built-in validated.

modules/gdscript/tests/scripts/runtime/features/does_not_override_temp_values.out

@@ -0,0 +1,2 @@
+GDTEST_OK
+done