Allow arbitrary const expressions in backed enums

[iluuu1994]

Closes GH-7821

Open issues:

• Every other run or so, ZSTR_HAS_CE_CACHE(class_name) returns true for "self" even though zend_alloc_ce_cache specifically guards for that. Sounds like memory corruption but I couldn't figure out where yet
• We probably need more forgiving error handling than zend_error_noreturn now that the errors are moved to runtime. All other errors in zend_do_link_class seem to be fatal so I think this is consistent.
• I changed the enum_backing_table to store the case directly but that might not actually work for internal enums (or we change zend_enum_add_case not to modify the backing table anymore and move that to after the constants have been initialized)
• Add tests for Can't use inherited constant value via 'self' in enum case value #7821 (comment)
• Can we add the backed_enum_table to the inheritance cache to avoid re-population on every request? That would require tracking dependencies of all classes in all expressions so likely not feasible.

[bwoebi]

Assigning a static property a constant expr of the wrong type gets you a TypeError (thus should be identical here). Everything else probably should just throw an Error.

In case zend_update_class_constants fails, you do nothing. This takes care of its own error handling deep down. Just return a failure. (E.g. zend_ast_evaluate directly throws exceptions.) Just ensure that the enum backed table building happens before the class is marked as linked. (i.e. move the zend_enum_build_backed_enum_table call up.)

[iluuu1994]

@nikic The backing table is now built when first accessing a constant or when first calling from/tryFrom. Let me know if you think this is acceptable.