Reflection: show the type of object constants used as default properties

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 phpGH-15902.
Closes phpGH-17781.

Author: DanielEScherzer

NEWS

@@ -51,6 +51,10 @@ PHP                                                                        NEWS
   . Partially fixed bug GH-17387 (Trivial crash in phpdbg lexer). (nielsdos)
   . Fix memory leak in phpdbg calling registered function. (nielsdos)
 
+- Reflection:
+  . Fixed bug GH-15902 (Core dumped in ext/reflection/php_reflection.c).
+    (DanielEScherzer)
+
 - Streams:
   . Fixed bug GH-17650 (realloc with size 0 in user_filters.c). (nielsdos)
   . Fix memory leak on overflow in _php_stream_scandir(). (nielsdos)