DCE shouldn't remove static calls to impure functions (fixes #10433)

[bendmorris]

There are some missing @:pure annotations in the std lib, which I noticed after fixing this as some unnecessary static vars would now be preserved. Fixed the ones I noticed.

Fixes #10433

[Simn]

Hmm, some of these might be controversial because of exceptions. For instance, isDirectory is documented to throw an exception in some cases, so something like this could be a pattern:

try {
    FileSystem.isDirectory(something);
    // do something else
} catch(e) {
    // handle case where it's not a directory
}

If the optimizer removes the call because it is considered pure, we end up with changed behavior.

[bendmorris]

Fair point; I pared the @:pure annotations down to be more conservative.

[Simn]

@kLabz Could you double check these @:pure(false) additions? I'm fine with merging this if there's no further potential problems.

[Simn]

Unfortunately, this PR never got updated and would need further work, so I'll have to close it.