Faster UnionType->isCallable()

[staabm]

before this PR

➜  phpstan-src git:(2.2.x) ✗ hyperfine 'php bin/phpstan analyze big-constant-string-union.php --debug -vv'
Benchmark 1: php bin/phpstan analyze big-constant-string-union.php --debug -vv
  Time (mean ± σ):     842.5 ms ±   2.8 ms    [User: 646.9 ms, System: 192.9 ms]
  Range (min … max):   839.2 ms … 848.4 ms    10 runs

after this PR

➜  phpstan-src git:(2.2.x) ✗ hyperfine 'php bin/phpstan analyze big-constant-string-union.php --debug -vv'
Benchmark 1: php bin/phpstan analyze big-constant-string-union.php --debug -vv
  Time (mean ± σ):     810.6 ms ±   2.5 ms    [User: 613.7 ms, System: 194.4 ms]
  Range (min … max):   807.8 ms … 814.3 ms    10 runs

refs phpstan/phpstan#15088

[phpstan-bot]

This pull request has been marked as ready for review.