Ensure trait_exists() always returns bool

[Ocramius]

Fixes #7478

As discussed in the upstream issue, trait_exists() always returns bool: while
it can return null when the arguments passed to it do not match (either no arguments, or
3 or more arguments), we do not support that scenario, as that already doesn't respect the
type signature of this function.

We cut to the point: always make it bool, which is the scenario that works under healthy
operational conditions.

Ref: Roave/BetterReflection#983 (comment)
Ref: https://psalm.dev/r/c41a43805d
Ref: #7478 (comment)
Ref: #7478 (comment)
Ref: https://3v4l.org/XpHmh

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/c41a43805d

<?php

function traitDoesIndeedExist(string $traitName): bool
{
    return trait_exists($traitName);
}

Psalm output (using commit 2e01e9b):

ERROR: NullableReturnStatement - 5:12 - The declared return type 'bool' for traitDoesIndeedExist is not nullable, but the function returns 'bool|null'

ERROR: InvalidNullableReturnType - 3:51 - The declared return type 'bool' for traitDoesIndeedExist is not nullable, but 'bool|null' contains null

[orklah]

Thanks!