[DeepClone] Suggest the extension when refusing a named callable, and decode its global-function references

[nicolas-grekas]

Q	A
Branch?	1.x
Bug fix?	no
New feature?	no
Deprecations?	no
Issues	-
License	MIT

Two related improvements to how the polyfill cooperates with the extension on first-class callables declared in constant expressions.

Refusal message. deepclone_to_array() refuses to serialize a closure over a named callable unless allow_named_closures is set. The message now warns that the option should only be enabled for trusted input, and points at the extension, which resolves more of these without the opt-in — a first-class callable declared in a constant expression, including cross-class references (Validators::check(...)) and global functions (strlen(...)), gets its declaring class recovered from reflection and is serialized as a declaration-site reference. Userland has no equivalent reflection hook, and this path only runs when the extension is absent, so the suggestion always applies.

deepclone_to_array(): serializing a closure over the named callable "strlen" requires enabling
the "allow_named_closures" option; do it only if you trust the input; alternatively, install the
"deepclone" extension, which can reference callables declared in constant expressions

Decoding the extension's global-function references. The extension can encode a reference to a global internal function declared in an attribute; such a function has no start line, so the reference is stored with line 0. The polyfill cannot produce these (no reflection hook), but must be able to decode them. ReflectionFunction::getStartLine() returns false for an internal function, so a line 0 reference was wrongly treated as stale; it is now normalized. With this, an extension-produced reference to a global internal function in an attribute round-trips through the polyfill.

The leading part of the refusal message is unchanged, so the tests (which match a substring) and any callers matching on it are unaffected.

Companion extension implementation: symfony/php-ext-deepclone#27