Export AST for default value strings in reflection

[nikic]

When dumping default values in ReflectionXXX::__toString(), for
expression initializers print the AST export instead of trying to
evaluate the expression. With the introduction of "new in
initializers" the result of the evaluation will commonly not be
printable at all, and "__toString" will throw an exception, which
is not particularly useful. Using the AST export also provides more
information on how the parameter was originally declared, e.g. it
preserves the fact that a certain constant was used.

[nikic]

@krakjoe @ramsey @patrickallaert This addresses an issue that Symfony encountered with PHP 8.1. While it was possible for Reflection __toString() to throw before (use of undefined constant in default value), with "new in initializers" in 8.1 this situation becomes much more common and makes __toString() output quite unreliable. Printing the original expression instead avoids any error conditions in this code. I also think it generally makes more sense for a debug dump.

Any objections to making this change?

[krakjoe]

No objections.

[ramsey]

No objections from me, either