[META] Fix strict type errors detected by phpstan

There are 271 errors identified in the code quality report, or 266 in the test report.

mstrelan → changed the visibility of the branch 11.x to hidden.

Not sure the best way to split this, so just started working on it. We can cherry-pick each fix to separate issues if we need to split it.

I don't think we need checkArgumentsPassedByReference here as that isn't affected by strict types (AFAIK).

Removed the bit about excluding tests. Tests call out to non-test code, and there can exist errors.