Faster DuplicateClassDeclarationRule

[staabm]

inspired by 11aebc1

since DuplicateClassDeclarationRule is the only caller of MemoizingReflector::reflectAllClasses, lets optimize the rule for its very specific use-case instead of memoizing only parts of the costs in MemoizingReflector.

DuplicateFunctionDeclarationRule will be optimized in a separate PR.

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before this PR:

➜  phpstan-src git:(class) hyperfine 'php -d memory_limit=450M bin/phpstan analyze -v --debug src/Analyser/Ignore' -i
Benchmark 1: php -d memory_limit=450M bin/phpstan analyze -v --debug src/Analyser/Ignore
  Time (mean ± σ):      2.547 s ±  0.019 s    [User: 2.204 s, System: 0.338 s]
  Range (min … max):    2.522 s …  2.583 s    10 runs

after this PR:

➜  phpstan-src git:(class) ✗ hyperfine 'php -d memory_limit=450M bin/phpstan analyze -v --debug src/Analyser/Ignore' -i
Benchmark 1: php -d memory_limit=450M bin/phpstan analyze -v --debug src/Analyser/Ignore
  Time (mean ± σ):      2.392 s ±  0.014 s    [User: 2.082 s, System: 0.304 s]
  Range (min … max):    2.376 s …  2.416 s    10 runs

[phpstan-bot]

This pull request has been marked as ready for review.

[staabm]

before this PR, the rule iterated all reflected classes, for every class found while analysis.

after the PR we built a map once and than lookup said map so we don't need to iterate the possible big array of all reflected classes over and over.