[mypyc] Fix native int default args in __init__ (#13910)

Avoid creating duplicate bitmap arguments for the constructor. These
would generate broken C with errors like these:

```
build/__native.c:122:74: error: redefinition of parameter ‘cpy_r___bitmap’
  122 | PyObject *CPyDef_Foo(int64_t cpy_r_xx, uint32_t cpy_r___bitmap, uint32_t cpy_r___bitmap)
```

Author: JukkaL

mypyc/irbuild/prepare.py

@@ -288,7 +288,8 @@ def prepare_class_def(
                 init_sig.ret_type,
             )
 
-        ctor_sig = FuncSignature(init_sig.args[1:], RInstance(ir))
+        last_arg = len(init_sig.args) - init_sig.num_bitmap_args
+        ctor_sig = FuncSignature(init_sig.args[1:last_arg], RInstance(ir))
         ir.ctor = FuncDecl(cdef.name, None, module_name, ctor_sig)
         mapper.func_to_decl[cdef.info] = ir.ctor
 

mypyc/test-data/run-i64.test

@@ -844,6 +844,37 @@ def test_class_method_default_args() -> None:
     assert dd.c(MAGIC) == MAGIC + 3
     assert dd.c(b=5) == 7
 
+class Init:
+    def __init__(self, x: i64 = 2, y: i64 = 5) -> None:
+        self.x = x
+        self.y = y
+
+def test_init_default_args() -> None:
+    o = Init()
+    assert o.x == 2
+    assert o.y == 5
+    o = Init(7, 8)
+    assert o.x == 7
+    assert o.y == 8
+    o = Init(4)
+    assert o.x == 4
+    assert o.y == 5
+    o = Init(MAGIC, MAGIC)
+    assert o.x == MAGIC
+    assert o.y == MAGIC
+    o = Init(3, MAGIC)
+    assert o.x == 3
+    assert o.y == MAGIC
+    o = Init(MAGIC, 11)
+    assert o.x == MAGIC
+    assert o.y == 11
+    o = Init(MAGIC)
+    assert o.x == MAGIC
+    assert o.y == 5
+    o = Init(y=MAGIC)
+    assert o.x == 2
+    assert o.y == MAGIC
+
 def kw_only(*, a: i64 = 1, b: int = 2, c: i64 = 3) -> i64:
     return a + b + c * 2