Skip special function optimization for redeclared disabled functions

As pointed out on GH-5817.

Completely remove disabled functions from function table

Currently, disabling a function only replaces the internal
function handler with one that throws a warning, and a few
places

Completely remove disabled functions from function table

Currently, disabling a function only replaces the internal
function handler with one that throws a warning, and a few
places in the engine special-case such functions, such as
function_exists. This leaves us with a Schrödinger's function,
which both does not exist (function_exists returns false) and
does exist (you cannot define a function with the same name).
In particular, this prevents the implementation of robust
polyfills, as reported in https://bugs.php.net/bug.php?id=79382:

if (!function_exists('getallheaders')) {
function getallheaders(...) { ... }
}

If getallheaders() is a disabled function, this code will break.

This patch changes disable_functions to remove the functions from
the function table completely. For all intents and purposes, it
will look like the function does not exist.

This also renders two bits of PHP functionality obsolete and thus
deprecated:

* ReflectionFunction::isDisabled(), as it will no longer be
possible to construct the ReflectionFunction of a disabled
function in the first place.
* get_defined_functions() with $exclude_disabled=false, as
get_defined_functions() now never returns disabled functions.

Fixed bug #79382.

Closes GH-5473.

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