When a property default is based on a global constant, show the type of the
default. Previously, `format_default_value()` assumed that non-scalar and
non-array defaults were always going to be `IS_CONSTANT_AST` pointers, and when
the AST expression had been evaluated and produced an object, depending on when
the `ReflectionClass` or `ReflectionProperty` instance had been created, the
default was shown as one of `callable` or `__CLASS__`.
Instead, if the default value is an object (`IS_OBJECT`), show the type of that
object.
Add test cases for both of the `callable` and `__CLASS__` cases to confirm that
they now properly show the type of the constant.
Closes GH-15902.
Closes GH-17781.
ca0414e64d