Fix array_key_exists() with all int literal keys

[brainlock]

(https://psalm.dev/r/a0e5cf2ff8)

When checking code like the following:

<?php

function checkNegated(string $key): void {
    $arr = [
        0 => "foo",
        1 => "bar",
    ];

    if (!array_key_exists($key, $arr)) {
        printf("not found\n");
    }
}

function check(string $key): void {
    $arr = [
        0 => "foo",
        1 => "bar",
    ];

    if (array_key_exists($key, $arr)) {
        printf("found\n");
    }
}

the if in checkNegated would cause:

ERROR: RedundantCondition - 9:10 - Type string for $key is never =int(0)

This happens when the array keys are all int literals, but the "needle"
is a string.

array_key_exists() uses a loose equality comparison, but the generated
assertions for this specific case
(AssertionFinder::getArrayKeyExistsAssertions) was generating strict
equality clauses. This commit fixes it by changing the generated clause
from = to ~.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/ceb5ad4bb1

<?php

$arr = [
	0 => "foo",
	1 => "bar",
];

$val_found = "1";
$val_not_found = "999";

if (array_key_exists($val_found, $arr)) {
    printf("found\n");
}

if (!array_key_exists($val_not_found, $arr)) {
    printf("not found\n");
}

Psalm output (using commit 257a1ca):

ERROR: RedundantCondition - 15:6 - Type "999" for $val_not_found is never =int(0)

ERROR: RedundantCondition - 15:6 - Type "999" for $val_not_found is never =int(1)

[brainlock]

The psalm.dev snippet I first linked could be confusing, so here's a new one to highlight the actual bug (it cannot know that the key is never in the array):

https://psalm.dev/r/d770e4c1c5

(I'll update the commit message as well)

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/d770e4c1c5

<?php

function checkNegated(string $key): void {
    $arr = [
	    0 => "foo",
	    1 => "bar",
    ];
    
    if (!array_key_exists($key, $arr)) {
    	printf("not found\n");
	}
}

function check(string $key): void {
    $arr = [
	    0 => "foo",
	    1 => "bar",
    ];
    
    if (array_key_exists($key, $arr)) {
    	printf("found\n");
	}
}

Psalm output (using commit 257a1ca):

ERROR: RedundantCondition - 9:10 - Type string for $key is never =int(0)

ERROR: RedundantCondition - 9:10 - Type string for $key is never =int(1)

[muglug]

Thanks!