hdl.ast: improve interaction of ValueCastable with custom __getattr__.

Avoid calling `__getattr__("_ValueCastable__lowered_to")` when
a ValueCastable has custom `__getattr__` implementation; this avoids
the need for downstream code to be aware of this implementataion
detail.

tests/test_hdl_ast.py

@@ -1060,6 +1060,18 @@ class MockValueCastableNoOverride(ValueCastable):
         pass
 
 
+class MockValueCastableCustomGetattr(ValueCastable):
+    def __init__(self):
+        pass
+
+    @ValueCastable.lowermethod
+    def as_value(self):
+        return Const(0)
+
+    def __getattr__(self, attr):
+        assert False
+
+
 class ValueCastableTestCase(FHDLTestCase):
     def test_not_decorated(self):
         with self.assertRaisesRegex(TypeError,
@@ -1083,6 +1095,10 @@ class ValueCastableTestCase(FHDLTestCase):
         sig3 = Value.cast(vc)
         self.assertIs(sig1, sig3)
 
+    def test_custom_getattr(self):
+        vc = MockValueCastableCustomGetattr()
+        vc.as_value() # shouldn't call __getattr__
+
 
 class SampleTestCase(FHDLTestCase):
     def test_const(self):