Faster DuplicateFunctionDeclarationRule

[staabm]

similar to #6038

the improvement is rather small because phpstan-src contains only few global functions.
in codebases with more functions it should safe even more. we also do this for consistency with DuplicateClassDeclarationRule

---

before the PR:

➜  phpstan-src git:(2.2.x) hyperfine 'php -d memory_limit=450M bin/phpstan analyze -v --debug src/Testing' -i
Benchmark 1: php -d memory_limit=450M bin/phpstan analyze -v --debug src/Testing
  Time (mean ± σ):      3.221 s ±  0.016 s    [User: 2.844 s, System: 0.372 s]
  Range (min … max):    3.201 s …  3.254 s    10 runs

after the PR:

➜  phpstan-src git:(funcs) hyperfine 'php -d memory_limit=450M bin/phpstan analyze -v --debug src/Testing' -i
Benchmark 1: php -d memory_limit=450M bin/phpstan analyze -v --debug src/Testing
  Time (mean ± σ):      3.202 s ±  0.007 s    [User: 2.837 s, System: 0.361 s]
  Range (min … max):    3.195 s …  3.218 s    10 runs

---

inspired by 11aebc1 which also memoizes allConstants. this method has not a single caller in one of the phpstan/* repos, therefore we don't tackle it

[phpstan-bot]

This pull request has been marked as ready for review.