Allow checking for functions

lib/private/app/codechecker.php

lib/private/app/codecheckvisitor.php

lib/private/app/deprecationcodechecker.php

.../data/app/code-checker/test-deprecated-constant-alias.php

...a/app/code-checker/test-deprecated-constant-sub-alias.php

tests/data/app/code-checker/test-deprecated-constant-sub.php

tests/data/app/code-checker/test-deprecated-constant.php

.../data/app/code-checker/test-deprecated-function-alias.php

...a/app/code-checker/test-deprecated-function-sub-alias.php

tests/data/app/code-checker/test-deprecated-function-sub.php

tests/data/app/code-checker/test-deprecated-function.php

tests/lib/app/codecheckvisitor.php

tests/lib/app/mock/codechecker.php