[mypyc] Fix decorated __new__ in native classes

[rheard]

Fixes mypyc/mypyc#1249.

A decorated method has no entry in ClassIR.method_decls, but the function below the decorators is in ClassIR.methods. For __new__ the code generator used both, and the result didn't work for any caller:

• The constructor and tp_new called the function below the decorators directly, so a native call skipped the decorators.
• The decorators were applied to the built-in __new__ function that calls tp_new. Setting the result on the class makes Python replace tp_new, which fails the safety check of that built-in function. Every call that isn't a native constructor call raised TypeError: C.__new__(C) is not safe, use object.__new__().
• If __new__ takes other arguments than __init__, the generated C didn't compile (too many arguments to function 'CPyDef_C_____new__').
• In a class with a built-in base class, __new__ in the type dict came from the method table, whose flags don't match the wrapper function of __new__. Instantiating the class crashed.